In the words of Edward MacLysaght, the first Chief Herald of Ireland ...

"The subject of Irish families is one in which much interest is evinced, but the popular books usually consulted and regarded as authoritative, particularly in America, are in fact unreliable. The inaccurate and misleading information thus imparted with cumulative effect is, however, much more deplorable in the armorial sphere than in the genealogical.
It is an indisputable fact that the publication presenting colour plates of Irish arms which is probably most widely consulted is no less than seventy per cent inaccurate, not only in mere detail, but often in points of primary importance and of an elementary kind. Apart from their many grotesque heraldic blunders the compilers of this work seem to have had a sort of rule of thumb; if they could not find arms for one Irish sept they looked for the name of another somewhat resembling it in sound: thus, for example, they coolly assigned the arms of Boylan to Boland. This frequently resulted in the arms of some purely English family being inserted in their book of 'Irish Arms' the Saxon Huggins being equated with O'Higgins, and so on. When this arbitrary method failed them they fell back on the arms of some great Irish sept. To quote one instance of this: Gleeson, Noonan and McFadden are all given the arms of O'Brien, though none of these septs had any connexion whatever with the O'Briens or with each other. Consequently many Americans of Irish descent are in good faith using erroneous and often English arms derived from the spurious source in question.
A certain cachet has been given to this because, in the more recent editions of O'Hart's Irish Pedigrees [published after the author's death - Ed], these same coloured plates have been inserted as if they were an integral part of O'Hart's book.
The serious genealogist uses O'Hart with caution, if at all, for he is a far from reliable authority except for the quite modern period. John O'Hart, however, undoubtedly did a vast amount of research, no matter how he used the information he acquired: I know that some of these errors of ascription can actually be traced to him, but it is surely an injustice to him that his well known name should be used as a cover for the propagation of false and often ludicrous heraldic statements."