Sliabh [sleeve] signifies a mountain; and according to O'Brien, it was sometimes applied to any heathland, whether mountain or plain. It occurs in the Zeuss MSS. in the old Irish form sliab, which glosses mons. The word in the anglicised form of slieve is applied to great numbers of the principal mountains in Ireland; and it is almost always followed by a limiting term, such as an adjective or a noun in the genitive case. For example, Slievesnaght, the name of a mountain in Innishowen, and of several others in different parts of the country, represents the Irish Sliabh-sneachta, the mountain of the snow; Slieve Anierin in Leitrim, Sliabh-an-iarainn, the mountain of the iron, in allusion to its well-known richness in iron ore; Slieve Bernagh in the east of Clare, gapped mountain.

This word is occasionally so very much disguised in modern names, that it is difficult to recognize it, and of such names I will give a few examples. There is a mountain west of Lough Arrow in Sligo, called Bricklieve, the proper Irish name of which is Breic-shliabh (Four Masters), speckled mountain, and the s has disappeared by aspiration. The same thing occurs in Finliff in Down, white mountain; in Gortinlieve in Donegal, the little field of the mountain; and in Beglieve in Cavan, small mountain. The parish of Killevy in Armagh, took its name from an old church situated at the foot of Slieve Gullion, which the Annalists usually call Cill-shleibhe, i. e. the church of the mountain, the pronunciation of which is well preserved in the modern spelling.

Sometimes the v sound is omitted altogether, and this often happens when the word comes in as a termination. Sleamaine in Wicklow is anglicised from Sliabh-meadhoin, middle mountain; Illaunslea in Kerry, the island of the mountain. Slemish in Antrim is well known as the mountain where St. Patrick passed his early days as a slave, herding swine; the full Irish name is Sliabh-Mis, the mountain of Mis, a woman's name; and there is another almost equally celebrated mountain in Kerry, of the same name, now called Slieve Mish.

In other cases both the s and v are lost, as for example in Crotlie or Cratlie, the name of several hills, Croit-shliabh, hump-backed mountain. In a great many cases the sound of s is changed to that of t by eclipse (p. 22), as in Ballintlea, the name of about fifteen townlands in the Munster and Leinster counties, Baile-an-tsleibhe, the town of the mountain; the same name as Ballintleva in Galway and Mayo, Ballintlevy in Westmeath, and Ballintlieve in Meath and Down; Baunatlea in the parish of Ballingaddy, Limerick, the baun or green field of the mountain.

The plural sleibhte [sleaty] appears in Sleaty, a celebrated church giving name to a village and parish in Queen's County. There can be no doubt as to the original form and meaning of this name, as it is written Sleibhte by all Irish authorities, and Colgan translates it Montes, i.e. mountains, The name must have been originally given to the church from its contiguity to the hills of Slieve Margy, as Killevy was called so from its proximity to Slieve Gullion.

Sleibhin [slayveen], a diminutive of sliabh, is applied to a little hill; in modern nomenclature it is usually made Sleveen, which is the name of a hill rising over Macroom in Cork, of a village in Waterford, and of nine townlands chiefly in the southern counties. Slevin in Roscommon is the same word; and Slevinagee in the same county, signifies the little mountain of the wind (gaeth).

The Origin and History of Irish Names of Places
P.W. Joyce, McGlashan & Gill, Dublin, 1871.