This may refer to Mahon (Mathgamain mac Cennétig), a southern chieftain and the elder brother of Brian Boru, who is believed by some to have fought a battle in 960 in the vicinity of Ballymahon at Shrule (in Irish Sruaith Fhuil, River of Blood) where he defeated O'Rourke of Cavan and laid claim to lands in the area.

[4] The earliest documentary evidence of Ballymahon is from the year 1578, when lands in the area were granted to the Dillon family, later Earls of Roscommon.

[5] By 1654, the maps of William Petty's Down Survey shows Ballymahon as a group of houses situated at the southern end of the present town, on the right bank of the River Inny.

From 1788 to 1853, the Roman Catholic bishops of Ardagh and Clonmacnoise resided in the town,[5] and the parish church served as a Pro-Cathedral until St Mel's Cathedral was built in Longford.

[citation needed] Brannigan Harbour, c.1 km from Ballymahon town, is on the Royal Canal and is a common point for boats and barges to stop and pass.

[10] Opposite St Catherine's Church is the old RIC police barracks built on the northern edge of what local tradition holds was a castle or fortress of the O'Farrell Clan, earthworks of which can still be seen off the Fairgreen/Thomand Lodge Road.

[citation needed] The Royal Canal links Ballymahon to Dublin, via several towns such as Mullingar and Maynooth, and to the River Shannon at Clondra.

[citation needed] Ballymahon is twinned with the two adjacent towns of Landévant and Landaul in the Morbihan region of Brittany in north-west France.