Hi, I didn't find the marriage record but did find 2 things that may be of help. In records for Killala & Achonry diocese, I found the marriage of Eliza's parents, John Whiteside to Sarah Dixon in 1802 (doesn't give exact date). Also, looking at the Tithe Applotment books which go from 1814-1838, there are no John Whitesides in Mayo.The only one I found is in 1833 and is listed at Templeboy,Augheras,county Sligo. I would lean more towards County Sligo as that is where they seem to be. If you go onto nli.ie,they have the Catholic parish registers online and for Killala the marriages go from 1815-1837 which would be the likely timeframe for Eliza and Thomas.Hope this helps,Kim