Sarah:
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
http://titheapplotmentbooks.nationalarchives.ie/reels/tab//004625691/004...
The same John, Michael and Patrick were again listed in the 1855 Grffiths Valuation head of household listing for Leitrim civil parish.
http://titheapplotmentbooks.nationalarchives.ie/reels/tab//004625691/004...
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.
Roger McDonnell