Leitrim's coastline is an intriguing concentration of Neolithic monuments - court, portal and wedge tombs and other unclassified megalithic structures abound. Recent evidence suggests that the deep blanket-bog which covers the Dartry mountains may also conceal many archaeological remains.

Ireland was once covered in woodland and five great forests are traditionally said to have stood in Leitrim. Many of these great forests were denuded for the making of charcoal for iron works.

The uplands of Leitrim were abandoned immediately after the Neolithic period, possibly because of soil erosion combined with climatic deterioration. It is not until the eighteenth-century A.D. that the marginal lands of these mountainsides attracted further settlement due to population pressure.

In ancient times Leitrim formed the western half of the Kingdom of Breifne.

This region was long influenced by the O'Rourke family of Dromahair.

Close ties initially existed with the O'Reilly clan in the eastern half of the kingdom. A split occurred in the 13th century and the kingdom was divided into East Breifne, now County Cavan, and West Breifne, now County Leitrim.

The Normans invaded south Leitrim in the 13th century but were defeated at the Battle of Áth an Chip in 1270. The Normans never recovered from this defeat.

The O’Rourke’s were famed for their hospitality and were celebrated patrons of poets and bards. However, it was their hospitality that led to their ruination and to the loss of their stronghold on the shores of Lough Gill.

When storms wrecked the Spanish Armada in 1588, a Spanish officer, Captain de Cuellar, was given shelter in the castle by Brian na Murtha O’Rourke. When the English found out, they arrested Brian, charged him with treason and hanged him at the Marble Arch in London.

As further punishment, the O’Rourke’s forfeited their lands. The estates became subsumed for redistribution to loyal English settlers. Much of the county was confiscated from its owners in 1620 and given to the “adventurers”, Villiers and Hamilton.

Leitrim suffered a massive population decline from the Great Famine of the 1840s until the 1990s. The county's population fell from about 160,000 in 1847 to 25,000 in 1996.

Seán Mac Diarmada the Irish republican political activist and revolutionary leader was born in county Leitrim. He was the second signatory of the Proclamation of the Irish Republic. He was executed in May 1916.