The Monastery of Clonmacnoise, also known as the "Seven Churches", was founded around 544 by the man who would later be known as one of the twelve apostles of Ireland, Saint Ciarán. The wealth of the monastery attracted the greed of Irish kings, Anglo-Normans and Vikings who plundered and burned it many times between 841 and 1204. The decline of the road bordering Clonmacnoise in favour of the one passing through Athlone around the 11th century marked the beginning of the decline of the site. In 1552, the English garrison of Athlone sacked the monastery. Three high crosses still stand on the site of the monastery, the "Cross of the North", the "Cross of the South" and the "Cross of the Scriptures", dating from the 9th to the 10th century.
High Crosses are the main surviving monumental works of Celtic art. They have existed in Ireland since the 6th century. Today these crosses are often found on monastic or ecclesiastical sites.
The site of the Kilclispeen Monastery dates back to the 5th-7th centuries. The monastery may have been founded by an unknown man, St Crispen. Nothing remains of this monastery, except for two high crosses from the 8th century, the "North Cross" and the "South Cross". Standing in the cemetery, the two crosses are 4 metres high, are made of sandstone and represent biblical scenes. There is also the base of a third cross on this monastic site.
Monasterboice (which means "Buithe Monastery") is the remains of a monastic settlement established in the 5th century by St. Buithe. Little is known about the monastery, except that it fell into ruin after the establishment of the nearby Cistercian abbey of Mellifont in 1142. The site includes the remains of two churches built in the 14th century or later and an old round tower, but it is especially famous for its high crosses. The three high crosses date from the 10th century and show biblical scenes.
Ardboe Monastery is an ancient monastery founded in 590 by Saint Colman. In the 16th century, the old church built by St. Colman gave way to the present church (in ruins). The entire monastery was destroyed by a fire in the 20th century. One of the main remains of the former monastery is the High Cross, the first one built in Ulster. There are a total of twenty-two panels of Old and New Testament scenes on all four sides of the cross.
St. Peter and St. Paul's Cathedral is an old cathedral, in use until 1643. It is the largest of Glendalough's seven churches and was built in several stages from the 10th century to the early 13th century. The oldest part of the ruin is the present nave. The choir and sacristy date from the late 12th and early 13th centuries. The market cross, which is now in the visitors' centre, was reconstructed in the 19th century from fragments scattered around the site. It may have originally stood opposite the west portal of the cathedral.
The monastery of Dysert O'Dea is said to have been founded by Saint Tola in the 8th century. Most of the ruins of the Romanesque structure visible today date mainly from the 12th century. The remains of the monastic site include the church of St. Tola, a round tower, St. Tola's well and a high cross, known as the Cross of St. Tola. The cross was knocked down by Cromwellian soldiers but repaired in 1683. In 1960 the cross was temporarily dismantled and shipped to Barcelona for an exhibition of Irish art.
St Columba's Church is a neo-gothic 19th- century church built on the site where, in the 6th century, the Drumcliff Monastery was founded. The last monks of the Drumcliff Monastery are mentioned as present in 1503, although the date that it was finally abandoned is unknown. Not much remains on the site of the monastery. The nearby high cross is the only example of its kind surviving in county Sligo. It is carved in the Urnes style. It has been dated tentatively on artistic grounds to the 9th or 10th century.