Roscommon comes from the Irish Ros meaning a wooded, gentle height and Comán, the first abbot and bishop of Roscommon who founded the first monastery there in 550 AD.

Rathcroghan (Irish: Ráth Cruachan, meaning "fort of Cruachan") is a complex of archaeological sites near Tulsk in County Roscommon. It is one of the six Royal Sites of Ireland.

It is identified as the site of Cruachan, the traditional capital of the Connachta, a term used to describe the prehistoric and early historic rulers of the western territory.

Rathcroghan is known as the home of Queen Medb  (Maeve) and was the seat of Kings of Connacht and then of the High Kings of Ireland.

This was the starting point of the Táin Bó Cúailnge, or Cattle Raid of Cooley, an epic tale in Irish mythology.

The north of the county was part of the Moylurg kingdom that covered an area of north-east Connacht and was ruled by the MacDermot clan until 1585.

The south of the county formed part of the territory of Uí Máine, which was ruled by the O’Kelly’s from the 8th century until the 16th century.

Roscommon Castle is a dramatic and imposing 13th Century Norman Castle.  It was built in 1269 by Robert de Ufford, Justiciar of Ireland, on lands he had seized from the Augustinian Priory.

The 'Cross of Cong', 'the Aghadoe crosier', 'shrine of the Book of Dimma' and the 'shrine of Manchan of Mohill' are grouped together as having been created by Mael Isu Bratain Ui Echach et al., at a Roscommon fine metal workshop.

Theophilus Sandford, a member of Oliver Cromwell's army in Ireland, received a large allocation of lands in Roscommon confiscated from the O'Connor family as part of the Act for the Settlement of Ireland 1652.

The National Famine Museum located at Strokestown Park, commemorates the Irish Potato Famine of 1845–49. The museum is located in what was once the family home of the Cromwellian "adventurer" family - the Pakenham Mahons - from the mid1600s until 1979. The estate was sold in 1979 as the last of the Mahons was afraid to live in Roscommon for fear of an IRA attack.

In February 1917 Count George Noble Plunkett won Sinn Féin’s first by-election in Roscommon North indicating the change occurring in Ireland following the execution of the leaders of the 1916 Rising.

Douglas Hyde, born in Frenchpark, County Roscommon, was a Gaelic scholar and writer and first president of Éire in 1938. Percy French was born at Cloonyquin House, near Tulsk in 1854.