Formed by the waters of the River Shannon above Athlone, Lough Ree lies between counties Roscommon, Longford, and Westmeath, 16 miles long and from 1 to 7 miles wide. The story of Lough Ree's formation duplicates that of Lough Neagh, with some substitutions. Eochu mac Maireda (sometimes called Ecca) had fallen in love with his stepmother, Ébliu (2), and together with his brother Ríb planned to form a new kingdom. Midir killed their horses and then offered them another one, presumably otherworldly, as compensation. When this horse urinated, it formed Lough Ree. Ríb, ModIr. Ríbh, may be the eponym of the Lough. Furbaide Ferbend killed Medb on an island in Lough Ree. In oral tradition Lough Ree was thought to contain both a monster and a submerged city. According to the medieval biography of St Mochuda, the Lough Ree monster once devoured a man; it was sighted in the lake as late as 1960. As for the submerged city, a bishop hearing that it contained a cathedral went to visit it but never returned.
See Life of St. Declan of Ardmore and Life of St Mochuda of Lismore, trans. Patrick Power. Irish Texts Society, 16 (London, 1914), 74–147.