Malachy Maguire and Paula Cain:
Welcome to Ireland Reaching Out!
I don't have good news for you. Presumably your Cain ancestors were RC, Unfortunately, the baptismal records for Dunhill parish start in April 1829 probably just after young William was born. https://registers.nli.ie/parishes/1106 Marriage records start in 1836. I did search on Roots Ireland just to make sure the marriage and baptism did not occur in another parish but did not find a record.
The Tithe records for the civil parish of Dunhill start are available in 1831 after the family left. By the way, no Cains but a few Keanes so it is possible that the spelling of the surname in Ireland was Keane or Kane.
The 1901 census for the Dunhill area only had one Cain (a seven year old) so likely none of your family is still there.
Sorry, I could not give you more positive news. We do have an XO Chronicles site where you can add the stories of your ancestors who left Ireland. https://www.irelandxo.com/ireland-xo/history-and-genealogy/ancestor-data...
Let me know if you have any questions.