Martin Fitzsimmons and Margaret MulHaul

My Great Grandffather was Robert Fitzsimmons he was born in Pennsylvania to parents who were born in Ireland. The names on his death certificate are Martin Fitzsimmons and Margaret Mulhaul. The certificate was signed by his wife I assume she would know. I cannot locate a copy of their marriage certificate although I did find where they were married. I have no idea of what age they were when the came to the US or if they married here or there. Robert died at age 48 and was a coal miner. He marriedIsabella Ridely and her daughter told us that when he died his extended family wanted to split up her & children (ranging in age from 17 to 2, the youngest was born 6 months after Robert died)with Robert because they did not think a young pregnant widow could support so many children. Isabella was English and had a child from a previous relationship which Robert raised, which they also apparently did not like. I know from this there must have been some relatives nearby but I cannot locate them., and do not know what part of Ireland the came from. If anyone has any information I would appreciate it.