How do I find out where in Ireland my Irish ancestors lived?

I am trying to discover where my great-great-great-great Irish grandparents, Charles Stockdale and Margaret (Sharpe) Stockdale, lived in Ireland. They were born in the early 1800s. Charles was actually born in Scotland but moved to Ireland when he was an infant. Charles and Margaret immigrated to America in 1854 after the famine, went through the port of New York aboard the "Southampton", and settled in New York and then Iowa. Does anybody know where or how I can trace back their origins and genealogy in Ireland, such as their Christenings, the county and parish they lived in, and their relatives in Ireland?