Much of Roscommon is underlain by limestone some of which especially in the south is at or near the surface, this contributes draining the land. In the north the soil is much deeper, and in places it was formed into the small rounded hills called drumlins, these were left behind by the receding ice sheets. There are peat bogs in poorly drained areas throughout the county.

Roscommon is almost entirely a limestone lowland. The limited areas of upland are in the extreme north. The Curlew Mountains along the Sligo border are sandstone. The Arigna area is part of a shale and sandstone plateau which is mainly in Leitrim.

The county employs a greater number of people in agriculture than any other county. Farming is mostly pastoral only about two percent of the land being used for arable farming, the crops grown are oats, barley and potatoes. The most fertile area of the county and indeed Connacht is in the south and center of the county. The most important farming activity is the rearing of beef cattle. In the south sheep are also important.

Manufacturing industries employ only about one quarter of the population. The main types are food processing, engineering, and metal industries and are located at Ballaghaderreen, Boyle, and Roscommon.