Hello Robert,
If you can find the marriage record for your mother and your father, regardless of whether it is valid or not, it should have his name and age, as well as probably where he was born.
Where were they married, and when? When you say that he was Irish, do you mean that he was born in Ireland? Or do you mean that his ancestors were Irish but he was born in the US (in other words, he is Irish-American)?
It is difficult to assist when it is not know how much or what information is available.