Hi,
killeenrevagh/Ballyglass are located very close to the border with Co Galway. While I cant comment on what the practice was in the 1850's, in modern times, people in that area would attend the catholic church in Ballyglass (not sure when it was built). There is also a graveyard in Ballyglass, in case that helps.
Another detail that might be relevant is that while killeenrevagh/Ballyglass are located in Co Mayo, the church in Ballyglass is linked to the modern parish of Milltown (Co Galway), but you wont find church records in the 1850's for Milltown, as it comes under "Addergoole and Liskeevy" for church records for that era.
In case you plan to look up the 1901 or the 1911 census, killeenrevagh/Ballyglass are both townlands in the Garrymore DED. Addergoole is the relevant DED for the Galway side of the border. Sometimes searching by DED is "safer" than searching for a specific surname in a particular townland, as there may be variations in the spellings of a surname. In addition, I've noticed that spelling mistakes have been made with some records when they were entered on to the electronic database.
Hope this helps.