Hi there & Merry Christmas,
Best way to start is with their immigration or shipping records, then use Roots Ireland, it's a bit expensive for downloading certificates, bu that's genealogy for you. A couple of tips regarding old Irish records, they sometime are written in Latin so if you don't get a result with the english version of the name try converting it to Latin and try that again. Also don't be surprised if a record "appears" to be in the wrong county, as several Irish villages straddle county borders so literally it depended what side of the street the registration took place on.
All the best with your search