Well there are no census records in Ireland after 1911 and that's not online yet. Do you know what your father's parents names were or where he was born? Any idea when his brothers and sister were born?

It sounds like they weren't in the town of Dundalk?

Basically you'd first need where he was born, parents names, and siblings names to try to figure out which sibling might have stayed behind. Then to find out what might have happened to him.