Frank - Because they were Catholic, even if they 'forgot' to register Thomas's birth , they would have had him baptised in the Catholic Church. If we can find the area in Belfast where they lived perhaps we could find the church.
As it happens a member of my family was also a stone mason during that time, and family history tells me that he worked on the Catholic Cathedral in Belfast. The family lived in Lilliput Street in the Dock area of Belfast.
RosemaryJoan