I've a feeling that all entries for births are 4Q, local register entries being sent up at the end of each year but I'd need to find confirmation of that.
I've got more detail now on the 1876 David and the 1878 Mary Jane. They are indeed siblings but there's something very odd about Mary Jane's birth and baptism. The informant in both cases at birth is Essie Porter and both births occurred in the townland of Corkley. Both children have the same mother yet David's father is Hans Porter and Mary Jane's is Thomas!! The family can be seen on the 1901 and 1911 censuses (look for Lena) and the husband clearly is Hans. Hans Porter and Lena Calaghan were married in 1874 at Castleblayney, Co. Monaghan which explains why the marriage is not on the GRONI index. The index entry is on ancestry.
There are only 4 David Porter born between 1874 and 1878. One is mine, son of Robert and Annie, one is the above son of Hans and Lena the other two were completely out of our area - 1874 Castlederg with mother's surname Russel and 1877 Belfast with a mother's surname Clarke.
According to the GRONI online index, the only Porter/Dilworth child born in 1876 is William born Lurgan district 31 May 1876. The entry on familysearch confirms the date of birth, gives the full name as William Henry Porter and the parents to be David Porter and Mary Jane Dilworth, father's birthplace Seagoe. Is Daveyh maybe recording the dates of registration rather than the actual dates of birth?
But there's another entry on familysearch for a John Porter born 22 May 1876 to David Porter and Ann Eliza Dilworth, father's birthplace Knockmenagh. There is no birth certificate entry for this child on GRONI online. I think we can only assume that the GRONI index is unfortunately full of errors :-(