Joss Naylor Lakeland Challenge:

The high points of the 32 counties of Ireland are covered by 26 peaks, ranging in height from 276 meters (Slieve Na Calliagh) in Meath to Carrauntoohill’s 1039. Several counties share a peak, Mount Leinster, for example, forms the high point of both Carlow and Wexford. Among these hills are some of Ireland’s most popular and appealing mountains, such as Errigal and Slieve Donard, and some that aren’t visited so often, such as Cupidstown Hill in Co. Kildare.

The "Head-to-Head" is a run between the southernmost (Mizen Head) and northernmost (Malin Head) points of Ireland. The exact route is up to the runner, but it is roughly 550km (mostly on roads). Eoin Keith discusses the route and history here.