I searched the Irish Family History Foundation's database. It has all surviving rcds for Co. Leitrim. Lots of early 19th century birth/marriage events in this Co. were never rcd'd or didn't survive.
Couldn't find either a B or M rcd for Peter McGuirl. There are death rcds for 1888 and 1903. Recording was civil by this time and comprehensive. A death rcd can be validated by looking at the birth date recorded thereon.
There are two B rcds for Thomas McGuirl in 1873. These seem redundant as one is in Murhaun parish, the other is in the Leitrim Civil District. A view of the detailed rcd for either, or both, may yield info on Thomas's parents.
Judging from the number of uses of various forenames, there were at least two families of McGuirls (+variants) living in Murhaun parish. Siblings of Thomas might include Ann, Bridget, Mariann, Mary Ann, James, Anthony, and Charles.