Dennis Sullivan b1855

I am researching my great grandad. I know he was from Cork, Ireland & that he moved to Brighton Sussex & married Sarah Ann Patching in 1880. I have their marriage cert, his father was of the same name, deceased at that time. I have all the census from that time forward but, know nothing of his life in Ireland or how he travelled here. If anyone can help I would be very grateful. I have tried Roots Ireland,Ancestry,Genes Reunited & others but, without success. Thanks Anne