I looked on the subscription site Roots Ireland. They have Tagoat RC baptismal records back to 1812. I found a record for a Catherine Rossiter baptized March 27 1812 in the Piercetown RC parish which is just to the northwest of Tagoat parish. Father was John Rossiter and mother was Margaret Stafford. They lived in Kilmacree townland in Kilmacree civil parish. There was also a record for Mary Rossiter baptized July 24 1815 also in Piercetown with the same parents and Kilmacree.
Then came William March 16 1819. Roots Ireland transcribed the father's name as Thomas but I looked at the register and it showed Jno for John. At that point, the family was living in Killiane which I believe is Killiane in Killiane civil parish. NOTE none of the baptismal records had sponsors with the surname Stafford or Rossiter
Piercetown and Tagoat marriage records start 1812 and 1853 respectively.
I think it will be difficult to find siblings to Thomas and Margaret. You may want to consider DNA testing perhaps ome matches will provide some clues..