From 'Reading The Irish Landscape' by Frank Mitchell and Michael Ryan
Galtymore, which is immediately west of Cahir in Co. Tipperary, is one of the few mountains in Ireland to exceed 900 metres or 3000 feet in altitude. The Galty mountains are a 20km long, 8km wide, range that rises abruptly out of neighbouring lowlands.
Geologically, the Galtys are an 'inlier' of older rocks (belonging to the Devonian and Silurian periods) protruding from the Carboniferous limestone lowlands. They have a east-north-east to west-south-west trend, reflecting origins during the Caledonian mountain-building period, with subsequent re-shaping during the later Variscan or Hercynian mountain-building.
Several other similar 'inliers' (less heavily modified in the Variscan) are to be found in other parts of north Munster and south Leinster. These include the large block of uplands on the Tipperary/ Limerick border (including Keeper Hill, the Slieve Felim mountains, the Silvermines and the Arra mountains). Other inliers form the Slieve Aughty (Clare/Galway border), Slieve Bernagh (east Clare) and Slieve Bloom (Laois/ Offaly) mountains.
A series of Corries along the northern flank of the Galtees marks the impact of locally-generated ice during the last Ice Age. As with the Wicklow Mountains and some other mountains, ice formed locally, and was separate to the great ice sheet that spead across the lowlands and surrounded the mountains. On the Galtees, however, the extent of the mountains and the volume of ice appears to have been insufficient for the formation of valley galciers and the consequent creation of U-shaped valleys.