Grand dad = Born in Ireland
Get his birth cert, marriage cert and death cert(if hes passed). Get your parents birth cert, and a copy of yours too.
If you cant get a copy of his birth cert, get in touch with the parish he grew up in. They will probably have a copy of his baptismal cert and are very accommodating.
so would you want to give up your US nationality for a Polish Passport?
Am I eligible or do I have to find the birth certificates and all of that stuff?
You wont need to pass a test, undergo health checks, undergo extensive vetting, process 100s of pages of paperwork and swear an oath of fidelity like America makes new wannabee citizens.
Proving you are who you say you are and that your paps actually was born here, is all that is required.