I searched the Irish Family History Foundation's database.
I found 2 M rcds for Peter in Co. Roscommon: 1827 & 1828. Since you don't have Bridget's maiden name, this is a calculated chance. One, the other, or neither of these may be for your ancestor. A view of the detailed rcd SHOULD show whether the bride's name was Bridget.
Only 20% of Roscommon's parishes were rcd'ng B events in 1811. Your chances of finding B rcds for the pair are slim indeeed.
Google the Foundation's name to lead you to this database.