If the family all left Ireland and moved to US then there might not be many records to find in Ireland. Civil registration started in 1864 (births, deaths, Catholic marriages) and 1845 (non-Catholic marriages). First conplete census is 1901. As with all other Irish research you really need to know a more exact place (parish if not townland) to start searching for church records, etc.
There may be clues in US records so you need to check everything there carefully. For example, U.S. death certificates (depending on the state and period) often list birthdates, birthplaces, names of parents, etc.