One of the four provinces in Ireland within it are the Counties of Mayo, Sligo, Leitrim, Roscommon and Galway.

Roscommon was the ancient capital of Connacht. The county that gave ancient Ireland its last High King and modern Ireland its first President.

Co. Roscommon is an inland county 100km in length from the top of the Arigna valley in the north to Shannonbridge in the south, with a breath of 64km at its greatest width.

The River Shannon is the boundary in the east and the River Suck in most of the west. Two-thirds of the county is bounded by water. In the north are the largest lakes - Lough Key, Lough Gara and Lough Boderg - with the great Lough Ree in the east.