Searching for Thomas O Neill B 1861 2 3 Ireland

I am searching for a Birth place for my Grandfather THOMAS O'NEILL b Ireland 1861/2/3 I have his Death certificate giving his age as 80 years in 1943.I have his Marriage certificate giving his age as 76 years in 1937 he was the son of PATRICK O'NEILL b about 1840, I think he was an ULSTER MAN Co ARMAGH.