Looking for Daniel ( Dan) Sheahan

Looking for details of my grandfather Daniel Sheahan, known as Dan.My father was born in 1911 in London, so Dan clearly left Ireland before then. Wife called Margaret, (Polly) who was English. Probably born around 1880 to 1890 at a guess. Would love to get his birth certficate or baptism certificate. Other than that, we know he was from County Cork, but not which parish, possibly pig farming family and definitely Roman Catholic. There are now four separate branches of 3 Sheahan generations in England, all wanting to know our history.