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Great job of researching your family.d RC marriages
Civil registration of births, deaths and RC marriages started in 1864.started in 1864.
It is possible the 1836 record is for your ancestor but it will be hard to verify.
No way to tell what happened to Elizabeth. I checked the free site www.irishgenealogy.ie and there are no Elizabeth Condron death records from 1864-1901 in the Mullingar registration district which not only covers Mullingar but also surrounding parishes.
She is not on the Mullingar RC death records pre-1864
The 1851 marriage record you located is a good lead but again no way to confirm.
Sligo is a good way from Westmeath. Did you know already that Patrick who married in 1892 had moved to Sligo because of his occupation?