The ancient Osraige inhabited the fertile land around the River Nore valley, occupying nearly all of what is modern County Kilkenny and the western half of neighbouring County Laois. To the west and south, Osraige was bounded by the River Suir and what is now Waterford Harbour; to the east, the watershed of the River Barrow marked the boundary with Leinster (including Gowran); to the north, it extended into and beyond the Slieve Bloom Mountains. These three principal rivers- the Nore, the Barrow, and the Suir, which unite just north of Waterford City, were collectively known as the Three Sisters (). Like many other Irish kingdoms, the tribal name of Osraighe also came to be applied to the territory they occupied; thus, wherever the Osraige dwelt became known as Osraige. The kingdom's most significant neighbours were the Loígis, Uí Ceinnselaig and Uí Bairrche of Leinster to the north and east and the Déisi, Eóganacht Chaisil and Éile of Munster to the south and west. Some of the highest points of land are Brandon Hill (County Kilkenny) and Arderin (on the Laois-Offaly border). The ancient Slige Dala road ran southwest through northern Osraige from the Hill of Tara towards Munster; which later gave its name to the medieval Ballaghmore Castle. Another ancient road, the Slighe Cualann cut into southeast Osraige west of present-day Ross, before turning south to present-day Waterford city.