St. Agnes' Church, Crumlin, was one of the first great era of suburban Churches in Dublin built under Archbishop Byrne (1921-1940). The firm of Ashlin and Coleman designed the Church, along with St. Teresa's, Donore Avenue (1922), St. Anne's, Shankill (1931), St. Columba's, Iona Road (1933), and Our Lady of Good Counsel, Mourne Road (begun in 1933, blessed in 1942).
Archbishop Byrne also oversaw the building of the Church of St. MacCullin, Lusk (1922), St. Brigid, Killester (1925), St. Vincent de Paul, Marino (1926), Garrison Church, Arbour Hill, (1927), Church of Our Lady of Perpetual Succour, Foxrock (1935), and Our Lady of the Rosary, Harold's Cross (begun in 1938, blessed 1940). In contrast to the Churches that were built under his successors, these were generally stone-built Churches in a very traditional style.
Temporary Churches were also needed in the new suburbs until a new Church could be built. As part of this great extension, Archbishop Byrne also blessed a tin Church at Portmarnock and a wooden Church at Kiltiernan.