County Mayo on Wikipedia.

Achill Sound (on Achill Island), Sisters of Mercy (R.C.). © Bill Henderson.

Ballina, Cathedral of St. Muredach (R.C.1827). Interior view. Link. St. Michael. All © John Balaam (2016).
Ballindine, St. Joseph (R.C., 1884). M 368 692. © Gerard Close (2017). Link.
Ballintubber (also Ballintober), the Abbey (R.C.). © Len Brankin. Link1. Link2.
Bohola, Church of the Immaculate Conception and St. Joseph (R.C.). It was built before 1895. This news article dates it to 1858. M 3075 9519. © Bill Henderson. Link. A map of 1838 shows an earlier church a very few yards south of (behind) the present church, and on a different alignment. It site is within the church's parking area, but not seen by Streetview. Shown on a map surveyed no later than 1838, less than ¾ of a mile away from Immaculate Conception, is Bohola Church, in the script used to denote an ancient place. By 1900 it was being labelled as In Ruins, and later maps, and an aerial view, suggest that little  or nothing of the church survives (though there appear to be some interesting lumps and bumps). It stood within a graveyard which survives, and it was seen (distantly) by Streetview in 2023. M 2986 9571.

Bunnacurry, St. Joseph. (R.C.). © Bill Henderson.

Charlestown, St. James (R.C.). G 477 018. © Gerard Close (2016). Link.
Claremorris, St. Colman (R.C.). Interior view. M 341 752. Both © Joseph Cantwell (2011). Another view, © Gerard Close (2017). The former CoI St. John was built in 1828, and was closed in 1963. It now serves as a library. M 341 752. © Gerard Close (2017).
Cong.
Crossmolina, St. Mary (CoI). © Bill Henderson.

Finney, Immaculate Conception (R.C.). © Len Brankin.
Foxford, St. Mary & St. Michael (R.C.). Church of Ireland. © Bill Henderson.

Glencorrib, Church of the Immaculate Conception (R.C.). © Bill Henderson.
Glenhest, St. Joseph. (R.C.). © Bill Henderson.

Kildonnet (on Achill Island), ruins of the old R.C. church. © Bill Henderson.
Kilkelly, St. Celsus (1960, R.C.). M 441 916. © Gerard Close (2016). Link.
Kiltamagh, Church of the Holy Family (1888, R.C.). M 341 892. © Bill Henderson. Another view, © Gerard Close (2016).
Knock, the Roman Catholic Basilica. © Bill Henderson. Another view, © Gerard Close (2014). St. Joseph at the Shrine (R.C.) (© Bill Henderson) - this is the apparition chapel, which encloses the rear wall of the original church, © Gerard Close (2014). Just visible behind St. Joseph is the old church still in use. © Graeme Harvey. Another view, © Gerard Close (2014). Blessed Sacrament Chapel, © Gerard Close (2014). Chapel of Reconciliation, © Gerard Close (2014).Map reference (for all) is M 394 827.

Louisburg, St. Patrick (R.C.). © Bill Henderson.

Mulranny, Catholic Church. © Bill Henderson.

Newport, St. Patrick (R.C.). © Bill Henderson.

Partry, St. Mary (R.C.). © Len Brankin.
Pollagh, St. Patrick (R.C.). © Bill Henderson.

Shanvaghera, St Anne (R.C.). © Graeme Harvey.
Straide, Catholic Church. © Bill Henderson.
Swinford, Church of Our Lady (R.C.). © Bill Henderson.

Turlough, Church of Ireland. © Bill Henderson.

Westport, St. Mary (R.C.). Holy Trinity (CoI). © Bill Henderson.