The Irish origin of the county name is cabhán, meaning "hollow" or "little hill", a description which will seem perfectly appropriate to anyone who has visited the county. Especially towards the Northwest, the main feature of the landscape is the proliferation of drumlins, oval mounds 80 to 100 feet high, which alternate with small lakes. The horizon is never more than a few hundred yards away, making the countryside feel small-scale and intimate.
In the Gaelic divisions which preceded the seventeenth century, Cavan, along with Leitrim, was part of the kingdom of Bréifne. Leitrim became known as Bréifne O'Rourke, while Cavan was Bréifne O'Reilly. The O'Reillys maintained their independence from English rule until the rebellions of the early 1600s. It was then incorporated into the province of Ulster, having previously been considered part of Connacht, and was included in the plantation of Ulster from 1608. Many English and Scots settled in the county at that time and subsequently, although O'Reilly, along with its variants, remained the single most common name.
Other surnames associated with the county include Brady, McGovern, Sheridan, Kiernan, McCabe, and Smith/McGowan.