Hello Mike,
Thank you for this. I've been putting a critical eye at my "evidence" and quite frankly it's wobbly. I think I must have drawn it from a 1782 Braddan will of Philip Killey, the summary of which is on the MNB:
"1782-2 E d KILLEY Phillip d 22 nov 1781;ch robt, philip, elizth,isabel ,susannah - wide [s/b "wife"] elizth admr;bro wm + bro-i-law john gelling supv;inv ".
[Philip KILLEY md Elizabeth CURPHY 12 Nov 1763 Braddan]

Philip Killey & Ann Clucas of Kk German's last two sons were William 1730 and Philip 1733 and Catharine was 1718. They seemed to fit to that will. But this will could easily belong to a different Philip Killey.

Catharine Gelling als Killey wife of John Gelling was listed as 94 in her 1806 burial, which works out to a birth of 1712. But if 1712 was correct, she was 50 when her last child was born. My theory to that was they might have exaggerated her age a bit from age 88 if this is the correct Catharine.

John & Catharine Gelling's fourth son was named Philip which added another tidbit of evidence.

I found the summary of Catherine Quilliam als Killey 1799 German on the MNB and it mentions her brother Philip Killey. That means the 1781 will of Philip Killey is not a match here.

Family Search identifies 5 different Catharine Killeys baptized 1716 to 1719 but no earlier ones. If you're confident you have the correct Catharine then the one who married John Gelling of Ballamillaghyn 1739 likely is one of the others. I need to research this further.