The Ryans are directly descended from Drona, who was fourth in descent from Cathair Mor, founder of the Drona tribe. The O'Riains, or anglicised Ryans, were chiefs of the tribe of Ui Drona (descendants of Drona), anglicised Idrone. In the eleventh century, the Chief of Ui Drona (descendants of Drona) took the surname of O'Ryan.
The O'Riains originally settled in Carlow and Kilkenny where they gave their name to the area. In the eleventh century the clan continued to possess the original stronghold of the family Ui Idrone i.e. much of present-day County Carlow and westward into County Kilkenny, in the vicinity of Graiguenamanagh.
In the late 12th century Dermot O'Ryan of Idrone granted land for the building of an abbey at Graiguenamanagh. The Cistercian Duiske Abbey at Graiguenamanagh is one of the oldest buildings associated with the Ryan family, and is still intact. It is now beautifully restored and is used as the Parish Church up to the present time.
The great historian, Geoffrey Keating, describes the Ryans as follows "A Sept most free and hospitable", and for their constant opposition to the English colonists, they had their possessions confiscated in the Cromwellian, and Williamites periods. The ancient Leighlin Cathedral, located to the north of Graiguenamanagh and about five miles west of Leighlinbridge was the major church in the Diocese in the centre of Idrone territory. The Cathedral, which is still used as a place of worship, was used for many events involving the Ryan family down through the centuries. A number of significant houses identified with the family are located near Goresbridge on the River Barrow, sixteen miles south of Carlow town.
In the 19th century, they were recorded as Ballinise, Aughtelkane Lodge and Ballycabus, and some are still occupied today. Seskin Ryan, a hamlet located to the south of Bagnelstown, County Carlow has strong family ties and now consists of a few houses. Although strictly outside the Idrone territory, Foulksrath Castle, eight miles north west of Kilkenny City, was owned by the Ryan family for many years, and is now used as a youth hostel.