The church of the Immaculate Conception (St Mary's) is situated in the townland of Tullybuck along the main Dublin-Derry road (N2) and was built between the years 1846 and 1865. The church is located in the heart of the local community, approximately half a mile north of Clontibret village and adjacent to Clontibret Community Centre (opened 1980). A small settlement has grown up around it.

The church building is cruciform in shape, consisting of a lengthy nave, short chancel and two transepts with an imposing tower to the western front. The stone is of local sandstone. St Mary's is an imposing and solid building that is the pride of not only the people of the immediate area but of the whole parish.

The three stained glass windows above the Sanctuary were erected in 1953. They were donated by Canon Eugene Coyle PP, Garrison, Co Fermanagh. A native of Threemilehouse, Fr Coyle was curate at St Mary's during the years 1916 to 1922 and played a prominent part in the struggle for national independence.

Fr Brendan Kieran SS.CC. was the resident curate at St Mary's from 1998 until his death on 30 December 2005. He is buried in St Mary's. He is the first resident priest of the parish to be interred here. Currently Fr Kenneth McCabe is the resident curate