Complex sentence: The Shannon Callows are areas of flat land along the shores of the River Shannon between Athlone where the river flows out of Lough Ree and Portumna where it enters Lough Derg in central Ireland.
Split into simpler sentences: The Shannon Callows are areas of flat land along the shores of the River Shannon. These areas lie between Athlone, where the river flows out of Lough Ree, and Portumna, where it enters Lough Derg, in central Ireland.