The surname Revington is rare in Ireland. There are only 4 B and 0 M records in the entire country. These four are all in Co. Laois: 1810, 1819, and 2-1883. The 1810 and 1819 rcds are possibilities for your ancestors.
Since there are no marriage rcds, I can't search for matching M rcds for John McGarity. The M rcd didn't survive, hasn't been transcribed, or they married in America.
For John McGarity B 1815 +/- 10 yrs, I found 58 rcds - far too many to easily summarize here. You will need a more exact DOB, parent(s) name(s), and/or placename(s) to pick which, if any, is the right one. Suggest you search the 1851 and 1861 Canadian censuses to find some tidbits of info.