September 1921, and civil unrest is simmering across Ireland. At a rally in Armagh City, Michael Collins, the MP for Armagh, gives embattled Northern Nationalists hope that they will be part of an independent free state. But when the Anglo-Irish Treaty is signed later that year, Catholics find themselves behind a new border and part of a reviled minority.

On Cashel Mountain, near the Armagh-Louth border, Bryan and Catherine Grant try to maintain a normal life for their four daughters. Meanwhile their son, Hughie, joins the escalating conflict between the 4th Northern Division of the IRA and the B specials. When feirce gun battles break out along the border  deadly reprisals follow each atrocity. Civilians are murdered in cold blood, women are voilated and homes are burnt to the ground, leaving families homeless.

As the 4th Northern is pushed south, the division becomes embroiled in the Civil War and must fight the forces of the new Irish Free State, brother against brother.

On the Border brongs to life the tragedy, terror And heroism of one of the most significant periods in Irish history by drawing heavily on actual events and the lived experience of a South Armagh mountain community that unexpectedly found itself on the frontline of the Irish Civil War.