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The monastic site of Clonmacnoise is situated in County Offaly, on the River Shannon south of Athlone.
Clonmacnoise was founded in 544 by St. Ciarán. Until the 9th century it had close associations with the kings of Connacht. The strategic location of the monastery helped it become a major centre of religion, learning, craftsmanship, and trade by the 9th century and together with Clonard it was the most famous in Ireland, visited by scholars from all over Europe. From the ninth until the eleventh century it was allied with the kings of Meath. Many of the high kings of Tara and Connacht were buried here.
Today, the site stands as a preserved ruin under the management of the Office of Public Works. An interpretive centre and facilities for visitors have been built around the site, which is open to the public.All three crosses have been moved to inside the centre and replaced by replicas outside. The graveyard surrounding the site continues to be in use.
Wikipedia for more details: Link

A National Monument in the Republic of Ireland is a structure or site, the preservation of which has been deemed to be of national importance and therefore worthy of state protection. If the land adjoining the monument is essential to protect it, this land may also be protected.
A list of the monuments can be found at Link