I don’t see the marriage either. There are 2 Margaret Mulgrew marriages in the period 1930 – 1940, both in Belfast, but neither married a Rocks. I suppose the options are:
- They didn’t marry
- They married outside Ireland
- For some reason the civil authorities didn’t get notified of their marriage.
Have you checked the children’s birth certificates to confirm the mother is shown as married ie "Margaret Rocks formerly Mulgrew?" If she’s just Margaret Mulgrew and the word "formerly" is deleted then they weren’t married.