How Do I Find Where IN Ireland My Ancestor Patrick Coghlan is from

i am researching my mothers mothers side of the family her. My Mothers grandfather was a jeremiah coghlan born in 1835 in south east london HIS father was patrick Coghlan,approx year of birth 1811 wife mary born in ireland approx year of birth 1813. The only census i can find him on is the 1851 english census where his place of birth is Ireland. at the time of Jeremiahs marrriage to my mothers grandmother he was listed as a widower ( cannot find his first marriage) his wife was a widow Elizabeth Collyer nee Mason, b 1851, they married in Poplar in a protestant church. Jeremiah's occupation at the time was stated as a mariner, family folklore says he was a deep sea diver for a telephone and cable company. i know this is all very vague but it is all i have. so when did they come to england? why did they come to england? My mum tells me Jeremiah was an Orange Man could that be a reason for the family to move to England?