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 and Manorhamilton, whose heraldic lion occupies the official county shield to this day. Close ties initially existed with the O’Reilly clan in the eastern half of the kingdom, however 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 in the 13th century and occupied the south of Breifne. Much of the county was confiscated from its owners in 1620 and given to Villiers and Hamilton. Their initial objective was to plant the county with English settlers. However, this proved unsuccessful. English Deputy Sir John Perrot had ordered the legal establishment of “Leitrim County” a half-century prior, in 1565. Perrott also demarked the current county borders around 1583. Five forests are traditionally said to have stood in Leitrim up till the 17th century.
Seán Mac Diarmada (27 January 1883 – 12 May 1916), also known as Seán MacDermott, bron in Corranmore, close to Kiltyclogher in County Leitrim was an Irish political activist and revolutionary leader in 1916 rising in Dublin. He was one of the seven leaders of the Easter Rising of 1916, who helped to organise as a member of the Military Committee of the Irish Republican Brotherhood (IRB) and was a signatory of the Proclamation of the Irish Republic. He was executed for his part in the Rising at the age of thirty-three
The Selton Hill Ambush took place on 11 March 1921, during the Irish War of Independence. An Irish Republican Army flying column was ambushed by members of the RIC Auxiliary Division at Selton Hill (aka Seltan Hill),in the Ballinmore area in County Leitrim. Six IRA officers of the Leitrim Brigade were killed.
Historic buildings are the “Carrick Castle”, the Workhouse and Famine Graveyard, Hatley Manor, St George’s Church of Ireland, Costello Chapel, Lough Rynn Estate, Fenagh Abbey, Margaret of New Orleans Homestead, Creevelea Abbey and Parkes Castle.

Working of the county’s rich deposits of iron ore began in the 15th century and continued until the mid 18th century. Coal mining became prominent in the 19th century to the east of Lough Allen in Sliabh an Iariann and also to the west in Arigna, on the Roscommon border. The last coal mine closed in July 1990 and there is now a visitor centre called Arigna mines. Sandstone was also quarried in the Glenfarne region.
William Butler Yeats spent the turn of the twentieth century fascinated with Lough Allen and much of Leitrim. Glencar Waterfall, 11 kilometres (7 miles) from Manorhamilton, inspired Yeats and is mentioned in his poem The Stolen Child.
Before the Irish Potato Famine of the mid 19th century county Leitrim had a population of over 115,000 which fell to 29,000 by the 1980s. In the twenty first century Leitrim’s population began to expand again and Leitrim now has the fastest growing population in Connacht around 32,000 people in total.