Johnson family

I am looking for information about my father to include his parents and possible relatives I might have.  His name is Alan Robert Johnson although I always heard his name was Robert Alan Johnson. When my mother died so did all the information on him.  He was born in 1926 or so.  I think if I can get a list of all the Johnsons with that name I can narrow it down myself.  Can anyone help? I know he and his parents came from Ireland.  I was born in New York as was many Irish Americans.  Thanks for any help. 

Sharon Johnson