I now have a marriage certificate and directory information

Facts:
Joseph O'Neill lived at 73 McDonnell street belfast from 1930 to 1974.
From 1930 to 1966 he was listed as a labourer. After this he was retired.
He had two children that I know of:
He had John O'Neill in 1923+/- 1 year. I have assumed this as he is listed as 18 years old when he married Alice Kelly in february 1941 in birmingham, England.
Searching through the 1911 census Joseph O'Neill is too popular to find a lead on. I think my best bet would be to find the birth certificate of John O'Neill.
I would be grateful if anyone would be able to suggest or help me in gathering more information regarding this family. I am hoping to visit groni in the near future as I am drawing a blank in accessing online information regarding the births/deaths of the O'Neill family.