Magh Sleacht, a plain in the northwest near the border village of Ballyconnell, was a hugely important 5th-century druidic centre. The principal Celtic deity, Crom Cruaich, diminished rather swiftly in significance as St Patrick set about spreading the Christian word.
In the 12th century, the Anglo-Normans tried to get a foothold in Cavan, but the landscape foiled them and the region remained under the control of the Gaelic O’Reilly clan. The O’Reillys ruled until the 16th century, when they joined the other Ulster lords to fight the Nine Years’ War (1594–1603) against the English and were defeated.
As part of the Ulster Plantation, Cavan was divided up among English and Scottish settlers. In the 1640s, taking advantage of England’s troubles, Owen Roe O’Neill led a rebellion against the settlers. O’Neill died in 1649 of suspected poisoning in Clough Oughter Castle near Cavan.
After the War of Independence in 1922, the Ulster counties of Cavan, Monaghan and Donegal were incorporated into the South. With the border so close, republicanism is strong here but with border points now removed you may not even notice crossing between the two.