Is it the marriage record of James/John McDonald to Ellen Barrett in 1882 that you are trying to find? If so, Barrett was a relatively unusual name in Cavan + Monaghan area compared to McDonald, I say this because 1901 census for Cavan had 16 Barretts whereas there was 515 McDonalds. The civil record may be easier to find than the church record and it will give you the name of the church. On www.familysearch.org under record search pilot for Ireland, Civil Registration Indexes, 1845 - 1958 there's an entry for Ellen Barrett, marriage in Cavan in 1882. I don't see a corresponding one for James or John McDonald but McDonald could have been spelled differently.
Where are you based? If in Dublin you can pick up this record in person from the GRO office in Irish Life Centre for 4euros otherwise you can write to the General Registry Office in Co. Roscommon. Send them the details of Ellen Barrett, Marriage, Jul - Sep 1882, Cavan, Vol 3, page no 67 and see if the record that comes back is your Ellen marrying James or John.