I searched the Irish Family History Foundation's database. It contains 11,500,000 surviving rcds for abt 60% of Irish counties. Roscommon is in it; however, it appears a number of parishes' rcds haven't been digitized. In any event, rcds B4 1840 are sparse.
For Michael Timon (+variants) M 1861 +/- 2 yrs, I found 1 M rcd in 1859, Tibohine-Fairymount parish. For Mary McDermott, I found 1 M rcd, same yr, same parish. These are likely your ancestors. A view of a detailed rcd will show whether the marriage is to the other. If so, it may list parents names etc. These can be used to validate B rcds.
For Michael Timon B 1836 (exact), I found 1 B rcd in Kilronan parish. This may or may not be your ancestor, but the rarity of the name and coincidence the year suggest it is. It can be validated if the parents names found on the marriage rcds match with those on the B rcd.
For Mary McDermott B 1841 (exact), I found 8 B rcds. Will leave it to you to sort out the parishes.