Mountains impinge upon the sea almost over the whole length, sometimes, as in Slieve League (county Donegal), immediately facing it with huge cliffs.
The principal groups, with their highest points, are the Mournes (Slieve Donard, 2796 ft.) and the Wicklow mountains (Lugnaquilla, 3 0 39) on the east; the Sperrins (Sawel, 2240) in the north; the Derryveagh group in the north-west (Errigal, 2466); the many groups or short ranges of Sligo, Mayo and Galway (reaching 1695 ft.
In the Twelve Pins of Connemara); in the south-west those of Kerry and Cork, where in Carrantuohill or Carntual (3414) the famous Macgillicuddy Reeks which beautify the environs of Killarney include the highest point in the island; and north-east from these, the Galtees of Tipperary (3018) and Slieve Bloom, the farthest inland of the important groups.
Lastly, rising in the Slieve Bloom or neighbouring mountains, the Suir, Nore and Barrow follow widely divergent courses to the south to unite in Waterford harbour.
They form an interesting and bleak moorland between Cookstown and Omagh, extending north-eastward into Slieve Gallion in county Londonderry, and consist fundamentally of mica-schist and gneiss, affected by earth-pressures, and invaded by granite near Lough Fee.