"Slieve" is an Anglicized form of the Irish word for mountain, sliabh. Bloom thinks in Calypso of some hills that bear his name ("Mine. Slieve Bloom"), and this range comes back in the company of some others near the end of Cyclops: "Slieve Aughty, Slieve Bernagh and Slieve Bloom."
The Slieve Bloom are among the oldest mountains in Europe, and as a consequence they are quite low. Millennia of erosion have reduced their height from some 3,700 meters (12,000 feet) to a maximum of 527 meters (1,729 feet). The Slieve Aughty, on the boundary of County Galway and County Clare some 40 or 50 miles to the west, consist of two ridges separated by the Owendallaigh river. They are no more imposing: their highest peak is 400 meters (1,314 feet).
The Mourne mountains of County Down, in Ulster, are more rugged, steep, and bare. Their fourth highest peak, Slieve Bernagh, has a rocky summit 727 meters (2,385 feet) high.