The town’s story begins in the 6th century when St Brendan allegedly founded a monastery here, which grew to become one of the most prestigious foundations in St Columcille’s federation of churches. Its strategic location at the crossroads of four ancient provinces; Munster, Meath, Leinster and Connacht; made it the perfect neutral ground for royal meetings and ecclesiastical synods, including notable gatherings in 697 and 1174 AD. The town developed southeast of its castle, following the pronounced meander of the River Camcor, which joins the Little Brosna northwest of the settlement.

The Anglo-Normans arrived in 1207, building a castle and establishing a settlement that thrived for about a century before the native O’Carrolls of Ely reclaimed the area by the mid-14th century. The O’Carroll stronghold, known as the ‘Black Castle’, served as one of the dynasty’s principal seats until they sold it to the Butler of Ormond in 1594. The Plantation of Ely O’Carroll in 1621 brought Laurence Parsons to Birr, granting him the lands, castle and fortifications, which he renamed Parsonstown. The Black Castle met its end in 1778 when it was demolished to make way for lawns and parkland; a 1691 map by Richards shows it as a rectangular fortress perched dramatically on the cliff edge above the Camcor River.

Today’s Birr Castle is a 19th-century creation by architect John Johnson, cleverly incorporating the bones of a 17th-century castle at its core. The current structure stands near the sites of both the medieval O’Carroll castle and an even earlier 12th-century Anglo-Norman motte, creating a remarkable palimpsest of Irish defensive architecture spanning nearly a millennium. The earliest documented attack on a castle here occurred in 1207, recorded in the Annals of Clonmacnoise, when Murchad Ua Briain destroyed the Anglo-Norman fortification, marking the beginning of centuries of contested ownership that would shape the town’s character.