Searching for info

I am in search formy fathers ancesters. Ireally do not know much about them. What i do know is, my father's name was William George Carrigan Jr.  Therefore my grandfather would be Sr. I know my father was born in Maryland in 1920's. I know his family came from Ireland and not so much else. My parents were divorced when i was small, and both have been deceased a long time. As i dig into this more imight be able to post more clues. My father had at least two sisters that i know of, both long gone now . One was Ethel, the other was Macy. Thank you.