The Slighe Mhor (pr. Shlee Vor) or Great Highway from Tara to Galway Bay, crossing the Shannon at Clonmacnoise, was officially opened AD. 123 by the King of Ireland.

Like the other four highways, it was skilfully planned, solidly constructed, and sufficiently wide to accommodate two chariots driving side by side.

Last July (July 1936. ed.) the Archdeacon of Clonmacnoise, Right Rev. Monsignor Langan, appealed to the public bodies to reconstruct that portion of the road leading immediately to St. Ciaran's city, so as to render it as safe for traffic as it was one thousand eight hundred years ago in the reign of King Cormac Mac Art. Centuries after its construction monasteries famous in ecclesiastical history flourished by the side of the great roadway, such as Clonard~ Durrow, Clonmacnoise, and that founded in The Name Carthach Or Carthage.

Rahan by "the greatest ornament of the Irish Church of his period" (Lanigan). namely, by St.Carthach (pronounced Kar-hadi) or Carthage or Mo Choda.

Another important geographical fact to bear in mind is that this road from Tara to Clonmacnoise passed through the ancient principality of Meath, which corresponded in territory with the present Diocese of Meath, plus the present Diocese of Clonmacnoise.