Clonfert Cathedral is the oldest continually operating church in Ireland. The cathedral is the remaining structure of the monastery founded by St. Brendan ("the Navigator") c. 557 CE. Like so many of the monasteries of Ireland it was raided and burned several times in the 11th and 12th centuries (i.e., 1016, 1164, 1179) only to be rebuilt after each attack. The final assault on the monastery occurred in 1541 leaving the monastery and most of the cathedral in ruins. Only the cathedral itself was restored after the sack of 1541, but not until 1664.

Tragically, very little, if any, of the original cathedral survives to the present. The west door with its unique Irish Romanesque arch is the oldest architectural feature and probably dates from the 12th century as it is dedicated to one Peter O'Moore who held the episcopal seat from 1161-1171. The bell tower and chancel were not added until 15th century.

The arch of the west door is decorated with one of the finest examples of mythic syncratism found anywhere in Ireland (or Western Europe). The general pattern of construction is a simple Romanesque arch with a triangular pediment. However, a close inspection of the arch reveals a much more complex sub-pattern. The arch is composed of five receding arches (composed of finely carved floral and solar motifs) supported by five finely decorated pillars on the north and south side of the door respectively. Resting on the main arch are five smaller arches which in turn support a cap of 15 stacked triangles and 10 recessed inverted triangles of the same proportions. Each of the base arches and each of the inverted triangular niches contains a single severed head harking back to the pre-christian Celtic cult of the severed head. There are other severed heads poking out here and there (some have been destroyed but it is clear where they were originally located) bringing the total number of heads to 25. So, throughout the structure of the doorway we find the number five and its multiples providing the structure and Celtic head veneration as the theme. What does it all mean? It's really anyone's guess.

The interior of the cathedral is studded with several unique architectural features as well, including the east window (dated to the 13th century) and several unusual carvings incorporated into the walls. Perhaps the most unusual is a depiction of a mermaid combing her hair. Most interpret this to refer to the voyage of St. Brendan where it is recounted by the haigiographers that he preached the gospel to the creatures of the sea. However, this is only speculation and there are many carvings that have no marine themes at all.

Perhaps the most unusual feature of the site is not actually within the sacred enclosure of the cathedral, but just outside the wall. On what was originally part of the monastery grounds there is a large grove of Yew and Beech trees. One of these trees has become a site for pilgrim sacrifice and one will find any number of secular and sacred objects left behind in devotion. But like the arch of the cathedral, what we really find is another example of the syncratism of the Celtic and Christian religions so prevalent throughout Ireland.