Trying to help in this thread. If your ancestors in America it is easy (due to our great documentation, census every 10 years) to reconstruct American Families backwards.
Ireland is an entirely different situation as the only census available are 1901 and 1911. FInd you people in their townlands there and you set. If not move back in time to Griffiths Valuation, or Tithe applotment.
The townland they came from is critical as you need the County, Townland, and Surname to find the right family line -- without that it is all guessing (and just because surname is same/similar means very little - think about How many Murphy lines there are 100s, 1000s, 10 or 1000s - so again, Surname, Townland, County, and search from there (have posting many free reseach sources on Sligo list)....