Can we see the contents of the letter?. Can you post an image.
"Where would I find the birth of Patrick Kelly Snr Born c1803
OR his marriage to Elizabeth Malone"
The more you know about the children of Patrick KELLY and Elizabeth MALONE, the better your chance to locate a marriage. The birthplace of a first child would suggest where they married.
Who are the witnesses for the marriages of Anne KELLY, and brother Patrick KELLY?