As you indicated, the Kenny/Kenney children were baptized in the RC parish of Leitrim in Co. Galway. I don't see a marriage record on the subscription site Roots Ireland. Possibly, they were married in a different RC parish which does not have records back to 1820 and earlier.
I looked at the 1843 Tithe listings for Leitrim civil parish and there was a John Kenney, Patrick Kenney and Michael Kennedy all listed in Kilmacrah townland
The same John, Michael and Patrick were again listed in the 1855 Grffiths Valuation head of household listing for Leitrim civil parish.
Possibly the John mentioned in both documents is your ancestor.
I did find a possible lead for the death of John Kenny. There was a John Kenny age 76 who died in the hospital at the Loughrea workhouse. No indication where John lived. He was shown as a widower and a labourer.
I did not find a good lead on a death record for Ellen.