Macgillycuddy's Reeks is Ireland's highest mountain range, and towering Carrauntoohil is the country's highest summit, at 1040m. There are several routes up, though even the most straightforward requires good hillwalking and navigation skills, while others are serious scrambling or rock-climbing routes.
The traditional route to the summit is via The Devil's Ladder, a gruelling trudge up a badly eroded gully path southwest of the loughs, which is unpleasantly loose and unstable. An alternative route ascends via Brother O'Shea's Gully (some rock scrambling and good route-finding ability required), a steep and challenging route through spectacular mountain scenery. The easiest descent is via the Zig-Zags to the east of the Devil's Ladder.
Experienced hillwalkers can follow the directions in Adrian Hendorff's guidebook, The Dingle, Iveragh & Beara Peninsulas: A Walking Guide. If you're in the slightest bit unsure, hire a guide – Kerry Climbing and Hidden Ireland Adventures both offer guided ascents of Carrauntoohil year-round, weather permitting (booking is essential).
Climbing Carrauntoohil should never be attempted without a map and compass (and the skills to use them), proper hillwalking boots, waterproofs and spare food and water. Use Harvey's 1:30,000 MacGillicuddy's Reeks Superwalker map, or the 1:25,000 Ordnance Survey Adventure Series map (MacGillicuddy's Reeks & Killarney National Park).