As you travel through each Irish village, there is normally one or two village churches: a Church of Ireland (Anglican) and a Roman Catholic Church. But the "big cities" have some beautiful cathedrals -- so Ellen is sharing a variety of photos of cathedrals or their interiors.
Celtic cross is a symbol that combines a cross with a ring surrounding the intersection. The symbol is associated with Celtic Christianity although it has older pre-Christian origins. Such crosses form a major part of Celtic art. A standing Celtic cross, made of stone and often richly ornamented, is called a High Cross or Irish Cross.