If you cannot find the place of origin for your Irish ancestors then it is always a good idea to look at other Irish immigrants who formed part of their community. The main reason for this is that immigrants to the United States and Canada sometimes traveled in groups and often participated in chain migration.
One book that provides an excellent example of this is Ireland to North America: Emigrants from West Cork by Joseph A. King (1994, K & K Publications, California). The publication also provides a case study of a feature of Irish immigration to North America that is less well studied - initial immigration to British North America (Canada) and then migration on into the American mid-west states. Emigrants from Schull Civil Parish, County Cork are the focus.
So, if you have any Driscoll, Mahoney, Donovan, Regan, Sullivan, Daly, Brien, Coghlan, McCarthy, Hickey, Kingston, Goggin, Sauntry or Lucey ancestors and you found the adult immigrant generation living by the Miramichi River in New Brunswick in the 1820-1840 time period and/or the next generation in Wisconsin, Minnesota, or Washington states, then they may very well have been from that part of south-west County Cork.