Valentia Lynches?

 My uncle did quite a bit of research to find that some of our Irish ancestors are from Valentia. I have been trying to find records for my gg grandfather, John  Lynch, born 26 May 1840. He came to the U.S. around 1860 and married Margaret Cuff, from County Wicklow, although I cannot find any marriage records in Pennsylvania, where they settled and where census records indicate a marriage around 1872, so not in Ireland. I believe that John's father was William Lynch and his mother was Catherine. Church records that I have searched online do not match the parents and birthdate. A sister Helen or Ellen Lynch was born around 1849.

With such a common name, can you help me how I may narrow it down or where I should search next?

Thank you!

Deborah