Knowing the county that your family hails from really does give such an important starting place in your search, as it can narrow down where to look and who to contact for more information.
As your father was living in the Republic of Ireland between 1921 and 1950, you may wish to contact the General Register Office
of Ireland. They would be able to help find official family documents, like births, marriages, adoptions, or deaths. You can read more about how to contact this office in our Helpful Research Tips
thread, which I cannot recommend enough as it is just full of wonderful advice.
Addition, I would also suggest taking a peek at our thread Tracing your Irish Ancestors: A Three-Step Guide
, which offers many great resources. Particularly, the 1911 census in the National Archives
may prove of use, if you know roughly when your grandfather was born. Beyond that, there are even more helpful tips and tricks which you may find useful.
I do hope that this helps and there's an amazing community of experts here who I am sure would be more than happy to lend you a hand as well!
Wishing you the best of luck,