My grandfather was born in Askeaton, CO. Limerick, in 1889 when it was still part of the UK. I’ve got copies of his arrival in NYC and his application for US citizenship and his citizenship certificate in Detroit. My cousin applied for and received an Irish passport. If I were still traveling, I think I’d apply for one as well. One must be able to show proof that one’s grandparent was born in Eire.
Ireland used to grant citizenship by virtue of one’s grandparent being born there. They changed it after 9-11. Now it has to be a parent who was born in Ireland. First you apply for citizenship via foreign birth, then you apply for the Irish passport.