In 1957 Edward MacLysaght published in the Irish Academic Press a series of papers entitled Irish Families (Their Names, Arms & Origins)
As a matter of interest to historians and genealogists I've extracted this part of his publication dealing with the distortion of Irish surnames.

Even in Ireland, where there is a genealogical tradition, it is quite common for people to be uncertain of their ancestry for more than three generations. Consequently a man in these circumstances whose name is, say, Collins or Rogers, to take two common in Ireland, cannot assert with certainty that he bears a native Irish surname. However, if he is a Collins, born and living in Dublin perhaps, whose people came from West Cork the odds are very strongly in favour of the true name being the Gaelic ? Coile?in. Smith, the commonest surname in England, comes high up in the Irish list - fifth in that given by Matheson. There can be no doubt that many of our Irish Smiths are the descendants of English settlers and traders, but it is equally probable that at least eighty per cent of the Smiths of County Cavan are of native stock, being MacGowans or O'Gowans who, under pressure of alien legislation or social influence, accepted the translated form and have used it ever since.

Many of the dual origin surnames are translations, like Smith and Oaks, or more often pseudo-translations such as Kidney and Bird. Some indeed of the latter are very far-fetched, even ridiculous, as for example the grotesque transformation of Mac Giolla Eoin into Monday from a fancied resemblance of the last part of that name to the Irish word "Luain".

So far we have been considering English names which in Ireland may conceal those of genuine Gaelic families. In a smaller number the converse obtains. Such names as Moore, Hart, Hayes and Boyle, which are, of course, genuinely Irish and are often regarded as exclusively so, are also found as indigenous surnames in England. So here again there is no certainty in the absence of an authentic pedigree, or at least of a well-founded tradition, as a guide. It has been pointed out for example that Guinness, which stout has made world-famous as an Irish name, and is in that case probably rightly derived from Magennis or MacGuinness of County Down, occurs in English records of some centuries ago in the rural county of Devonshire.

Probably the most reliable and scholarly work on English surnames is that of Professor Weekley. Yet he includes in his lists, without any mention of Ireland, several like Geary, Garvin, Grennan and Quigley: typical Gaelic-Irish surnames which, while they are no doubt occasionally found with the French or Anglo-Saxon background he indicates, when met in England at the present time are much more likely to have been brought there by Irish immigrants.

Apart from these surnames of possible English origin there are many indisputably Irish surnames not indigenous in England which assumed in their anglicised form a completely English appearance. What, for example, could be more English in appearance than Gleeson, Buggy, Cashman, Halfpenny and Doolady, to cite only a few examples. All of these are genuine Gaelic surnames and surprisingly numerous.

Once again the converse of this is also true. No one unacquainted with the subject would doubt that such very Irish sounding names as Gernon, Laffan, Gogan, Henebry and Tallon, and even O'Dell, all quite common in Ireland, are Irish, yet none of them is of Gaelic origin. This list, however, is not so long.

Some Gaelic surnames in their modern anglicised form have acquired an equally un-Irish guise but have a foreign rather than an English look. Coen, a variant of Coyne, and Levy, a common abbreviation of Dunlevy, suggest the Jew; I know a Lomasney who is always refuting the erroneous belief that he is of French origin, and I expect Lavelles and even Delargys and Delahuntys may have the same difficulty; Hederman and Hessian have rather a German sound, while Nihil, well known in County Clare, and Melia, synonym of O'Malley, might be Latin words. Most of this class, however, are occasional variants, such as Gna and Gina for (Mac) Kenna or Manasses for Mannix, or rare surnames like Schaill, Thulis and Gaussen.

In some cases the anglicisation process has had very unfortunate results. The beautiful name Mac Giolla ?osa, for example, usually rendered as MacAleese, takes the form MacLice in some places. The picturesque and heroic ? Dathlaoich in County Galway ridiculously becomes Dolly and the equally distinguished Sealbhaigh which is anglicised Shelly in its homeland (Co. Cork) is Shallow in Co. Tipperary. Schoolboys of these families, unless they use the Irish form, need no nicknames; Grimes, too, is a miserable substitute for its Gaelic counterpart Greachain, which has also Grehan as a more euphonious anglicised form.

These corruptions, of course, are due to the influence of the English language, the spread of which in Ireland was contemporary with the subjection and eclipse of the old Catho1ic Irish nation: names of tenants were inscribed in rentals by strangers brought in to act as clerks, who attempted to write phonetically what they regarded as outlandish names; in the same way Gaelic speaking litigants, deponents and witnesses in law cases were arbitrarily dubbed this and that at the whim of the recording official. It was not until the nineteenth century that uniformity in the spelling of names began to be observed, but the seventeenth century was the period during which our surnames assumed approximately the forms ordinarily in use in Ireland today.

The corruptions we have noticed above have been cited as examples of the tendency to give Irish names an English appearance. Most of them have at least some phonetic resemblance to their originals or else were frankly translations or supposed translations. There is, too, a large class of Irish surnames anglicised in a way which makes them quite unrecognisable. Often these distortions are aesthetically most unpleasing, as Mucklebreed for Mac Giolla Bride and Gerty for Mag Oireachtaigh.

Citing only official registrations with the Registrar-General, Matheson notes a particularly flagrant example, viz. a family of O'Hagans in County Dublin who have actually become Hog, which in the absence of his testimony one would naturally assume to be simply the well-known English surname of Hogg (O'Hagan is unlucky in this respect. According to Woulfe the very English and plebian-sounding Huggins is one of its synonyms in Ireland). Rather less cacophonous is Ratty for Hanratty. Forker for Farquhar (in County Down) may perhaps be regarded as comparable to the contraction in England of Cholmondeley to Chumley and Featherstonehaugh to Fanshawe in less aristocratic circles, these of course being phonetic spellings. The most curious instances of phonetic abbreviation recorded by Matheson is the birth registration of a Dalzell child at Dundalk "tout court" as "D.L.", that being the peculiar pronunciation of Dalzell in its native Scotland. The commonest of all Irish surnames, though not aesthetically objectionable, is a good illustration of decadence, for Murphy is a far cry from MacMurrough and 0'Morchoe, as is Dunphy from its synonym O'Donoghue. My own name, which I am glad to say is a true Dalcassian (Co. Clare) one, is an excellent example of the distortion we are considering, for no one would readily connect MacLysaght, especially when shorn of its Mac, with Mac Giolla Iasachta. The seventeenth century officials did at first render it as McGillysaghta, etc. in documents in English, but this proved too much of a mouthful to last long.

This name is also an example of that fairly numerous class in which the initial letter (excluding the prefix) is misleading. The L of Lysaght and of Leland derives from the gioLLa. The origina1 L of Lally on the other hand is to be found in the MaoL of the original. In the same way the C of Clancy, the K of Keogh and the Q of Quaid are from MaC; the G of Gaynor and Gorevan from the MaC prefix (Mag is a form of Mac frequently used with names beginning with a vowel), while the Il of Ilhenny can again be traced to the gIOLla of the Gae1ic form.

Another tendency in the anglicisation of Irish surnames is the absorption of uncommon names in common ones. Blowick, for example, tends to become Blake, Kildellan is merged in Connellan, Cormican in McCormick, Sullahan in Sullivan, Kehilly and Kilkelly in Ke1ly, and so on. Certain well-known family names such as Courtney, Conway and Leonard have gobbled up in the course of time, not one, but half a dozen or more minor ones. We must presume that this was a result of the general Gaelic depression, part of the same indifference and hopelessness which acquiesced in the lopping off of the Mac and O from so many old Irish surnames.

I have said that the mutilation and corruption of Irish surnames took place in the seventeenth and to a lesser extent in the eighteenth centuries. It must be admitted, however, that even today, fifty years after the foundation of the Gaelic League, the gradual re-gaelicization of names resulting from its influence is to some extent counterbalanced by the opposing forces of de-nationalisation. This is found more in pronunciation than in spelling: though even in this official registration age pronunciation does tend to affect spelling. A notable example of what I have in mind is the internal H. The English seem unable to cope with this sound which presents no difficulty to an Irishman: for Mahony they say Mah-ney (or, as they would write it, Marney, since the internal R is also dead in England). Now Dublin and suburbs with over 650,000 people contains more than one fifth of the population of the Republic and one seventh of the whole country; and Dublin for a11 its genuine political nationalism is in most ways more English, or perhaps it would be more accurate to say, more cosmopolitan, in character. The contrast between Connacht and Dublin is as marked as that between Dublin and England. Of course the good old Dublin accent has lost none of its distinctive raciness, but it is only to be heard in the mouths of one section of the citizens. The gradual disappearance of regional Irish accents is much to be deplored: it is due to a number of causes including the B.B.C., the cinema, the much increased intercourse with England resulting from the recent mass emigration to that country, and perhaps I may add the "refinement" aimed at in convent education. However, I must not allow myself to go off at a tangent on this interesting topic, which is irrelevant except in so far as it is concerned with the pronunciation of surnames.

In America the distortion of the name Mahony takes a different form, for it is often mispronounced Ma-honey, just as the wrong vowel is stressed in Carmody and Connell. In Ireland one does not hear Ma(r)ney for Mahony or Clossey for Cloghessy, but boggling at the internal H has come to Dublin now. I know a family in Dublin named Fihilly: the parents insist quite rightly that there are three syllables in the word, but the younger generation are content to answer to "Feeley" and so pronounce the name themselves; Gallaghers in Sydney, after a long losing battle with Australian philistinism, have had to accept "Gallagger" with the best grace they could. This, however, may be partly due to the ocular influence of the middle G. There is another difference in these two cases, besides the fact that the Fihilly deterioration took place in Ireland itself: Feeley has actually become a recognised way of spelling that name. Similarly there are Dawneys who were originally Doheny.

The surnames Hehir and Cahir in Thomond are still dissyllables, but the latter when denoting the town of that name in Co. Tipperary has become immutably "Care". This again prompts a long digression on place names: but that subject, so full of pitfalls for all but the most learned, would be out of place in this text.

The internal H is not the only stumbling-block for English people and anglicised Dubliners. They pronounce Linnane as Linnayne and Kissane Kissayne. Our "ane" sound, which is intermediate between the English "Anne" and "aunt", is not heard in English speech. Similarly O'Dea is called O'Dee. These emasculated pronunciations sound like affectation to people who come from the places where those names originated and still abound. This is not to deny that there is actually a name O'Dee, but that is not a Clare name, as O'Dea emphatically is.

Some English inspired innovations fortunately do not last. During the first World War a neighbour of mine in Co. Clare named Minogue joined the British army; in due course he returned as Capt. Minogue - Captain "Minnow-gew", if you please, not "Minnoge"! He may have got the idea from the mistake of a fellow soldier but he adopted the monstrosity and even insisted on it.

One of the most irritating of the examples of capitulation to English influence is the adoption of the essentially Saxon termination "ham" for the Irish "ahan", "ann", etc. This is not confined to surnames: the Gaelic word "banbh", called bonnive in English in the less anglicised counties, is bonham in most places. Rathfarnham, recte Rathfarnnan, is the best known place so anglicised; while on our own ground we have the very English-looking Markham, a Clare surname of which the normal version should be, and indeed formerly was, Markahan (cf. the place name Ballymarkahan in Co. Clare).

In the same way, but less noticeably, the final S so dear to English tongues degaelicizes Higgin(s), while the addition of an unnecessary D has somewhat the same effect on Boland. This D seems to have been a matter of chance for Noland is almost as rare as Bolan.

Quite often the anglicisation of a Gaelic surname resulted in the adoption in English, whether consciously or not, of one which carried a certain social cachet like D'Evelyn for the usual Devlin, Molyneux for Mulligan or Delacour for Dilloughery. Montague for MacTadgh or Mactague probably arose in the same way, the sound Montag at some period giving way to Montagew through the ocular influence of the spelling. The cognate Minnogew for Minogue was just "swank". We may assume that the good captain's descendants have gone back to plain Minnoge, as it is only a matter of pronunciation in their case.

There are other examples of this tendency which cannot be shed so easily. When Mulvihil has thus become Melville and Loughnane Loftus, resumption of the true patronymic necessitates (in practice, though not in strict law) certain legal formalities. - am told that there are people whose name was originally Mullins (Maolain) using the form de Moleyns. I have not met a case myself. According to Burke's peerage the best known family of the name, the head of which is Lord Ventry, are not true Irish Mullinses at all, and they presumably had justification for assuming the form de Moleyns in place of Mullins, a step which they took in 1841.

Some people with Mac names insist on the Mac being written in full, others prefer Mc, and formerly M' was quite usual. It is hard to understand why any objection should be taken to Mc or even M', since these are simply abbreviations of Mac. The practice of some indexers, notably in the Century Cyclopaedia of Names, of differentiating between Mac and Mc is to be deplored, since the reader must seek the name he wants in two places - in the Macs, which are interspersed among such words as Maccabees and Macedonia, and in the Mcs many pages further on. It is impossible to differentiate satisfactorily. Take MacGillycuddy for example: it appears in the work in question as MacGillycuddy's Reeks, yet the Chief of the Name always subscribes himself McGillycuddy of the Reeks. The idea that Mac is Irish and Mc Scottish is just another popular error. Mcc, however, may fairly be called an affectation, being merely the perpetuation of a seventeenth century scribe's slip of the pen.

The most prevalent of peculiarities in the spelling of names - the use of two small f's for a capital F - would seem to have arisen not through snobbery but from ignorance: the originators of this now carefully treasured blunder were probably unaware of the fact that in seventeenth century documents the normal way of writing F was ff, a symbol almost indistinguishable from f f.

The Irish Information website an excellent website with many free resources also has a page on the Origin of Irish Names