Derg, Lough (lŏkh dĕrg) [key], in Ireland. 1 Expansion of the Shannon River, 23 mi (37 km) long and 1 to 5 mi (1.6–8 km) wide, W central Republic of Ireland. On the lake is the republic's first (1927) major hydroelectric power plant, with an 85,000-kW capacity. On Holy Island or Iniscaltra are ruins of churches and a round tower. 2 Lake, c.20 sq mi (50 sq km), Co. Donegal, NW Republic of Ireland. Station Island, traditional site of Saint Patrick's Purgatory, the popular name for the saint's vision of Purgatory, is a famous place of pilgrimage. Saints' Island has ruins of a monastery.