Ireland is a limestone plateau rimmed by coastal highlands of varying geological structure. The central plain area, characterized by many lakes, bogs, and scattered low ridges, averages about 90 m (300 ft) above sea level. Principal mountain ranges include the Wicklow Mountains in the east and Macgillycuddy's Reeks in the southwest. The highest peaks are Carrantuohill (1,041 m/3,414 ft) and Mt. Brandon (953 m/3,127 ft), near Killarney, and, 64 km (40 mi) south of Dublin, Lugnaquillia (926 m/3,039 ft).
The coastline, 1,448 km (900 mi) long, is heavily indented along the south and west coasts where the ranges of Donegal, Mayo, and Munster end in bold headlands and rocky islands, forming long, narrow fjordlike inlets or wide-mouthed bays. On the southern coast, drowned river channels have created deep natural harbors. The east coast has few good harbors.
Most important of the many rivers is the Shannon, which rises in the mountains along the Ulster border and drains the central plain as it flows 370 km (230 mi) to the Atlantic, into which it empties through a wide estuary nearly 110 km (70 mi) long. Other important rivers are the Boyne, Suir, Liffey, Slaney, Barrow, Blackwater, Lee, and Nore.