The O'Donnell dynasty (Irish: Ó Dónaill or Ó Domhnaill, Ó Doṁnaill or Ua Domaill; meaning "descendant of Dónal") were the dominant Irish clan of the kingdom of Tyrconnell in Ulster in the north of medieval and early modern Ireland.

Like the family of O'Neill, that of O'Donnell of Tyrconnell was of the Uí Néill, i.e. descended from Niall of the Nine Hostages, High King of Ireland at the beginning of the 5th century; the O'Neill, or Cenél nEógain, tracing their pedigree to Eógan mac Néill, and the O'Donnells, or Cenél Conaill, to Conall Gulban, both sons of Niall. Conall was baptised by St. Patrick.

The Roman Emperor Constantine the Great converted to Christianity after a vision before the famous Battle of the Milvian Bridge, having seen a chi-rho in the sky, and thence the motto In Hoc Signo Vinces, telling him he would be victorious with the sign of the cross. The chi-rho was adopted on a banner, the labarum, upheld on a vexillum, which resembled a Christian cross, and in time the motto became associated with the Cross all over Europe. Legend has it that St. Patrick struck the shield of Conall, son of King Niall of the Nine Hostages, with his crosier, called Bachal Isu (the staff of Jesus) inscribing thereon a sign of the cross and told him the same, and baptized him. According to the Life and Acts of Saint Patrick (chapter 138), commissioned by Sir John de Courcy and written by Jocelyn of Furness (c. 1185 AD), St. Patrick took his staff, known as the staff of Jesus, or Bacall Iosa, and struck the shield of Prince Conall, rendering a sign of the Cross on it, “et mox cum baculo suo, qui baculus Jesu dicebatur Crucis signum ejus scuto impressit, asserens neminem de stirpe ejus in bello vincendum qui signum illud”, and thus indicating that he and his offspring would henceforth be victorious in battle if they followed that sign This legend is also described several centuries later in the Lebhar Inghine i Dhomhnaill. His land became Tír Chonaill, Tyrconnell, the land of Conall.

Conall's Constantinian shield, and this motto, have been the main O’Donnell arms in various forms, through the centuries. The motto also appears prominently placed as a motto on a ribbon unfurled with a passion cross to its left, beneath a window over the Scala Regia, adjacent to Bernini's equestrian statue of Emperor Constantine, in the Vatican. Emperors and other monarchs, having paid respects to the Pope, descended the Scala Regia, and would observe the light shining down through the window, with the motto, reminiscent of Constantine's vision, and be reminded to follow the Cross. They would thence turn right into the atrium of St. Peter's Basilica, ostensibly so inspired. In an earlier version (before Bernini's renovations in the mid-17th century), something similar may have resonated with and been observed by Prince Rory O'Donnell, 1st Earl of Tyrconnell following his visit to Pope Paul V (at the Palazzo Quirinale) in Rome, just prior to his death in 1608. It would certainly have resonated with and been observed by Cardinal Patrick O'Donnell.

Tyrconnell, the territory named after the Cenel Conaill, is the vast territory where the O'Donnells held sway, comprised the greater part of the modern county of Donegal except the peninsula of Inishowen. But it also included areas outside Donegal, such as the baronies of Carbury in County Sligo, Rosclogher in County Leitrim, and Magheraboy and Firlurg in County Fermanagh, and part of southern County Londonderry, hence it straddled the modern Republic of Ireland and also part of Northern Ireland in the UK. The jewel in the O'Donnell crown was Donegal Castle, one of seven O'Donnell castles, and now a national monument restored in the 1990s. Tyrconnell also, therefore, bordered on territory ruled by the O'Neills of Tyrone, who were periodically attempting to assert their claim of supremacy over it, and hence the history of the O'Donnells is, for the most part, a record of clan warfare with their powerful neighbours, and of their own efforts to make good their claims to the overlordship of northern Connacht, and a wider swathe of Ulster. Nonetheless, Tyrconnell existed for a period as an independent kingdom, recognised by King Henry III of England.

Gofraidh Ó Domhnaill, the first chieftain, was son of Domhnall Mór Ó Domhnaill. In 1257, Gofraidh was victorious when he went to battle at Creadran-Cille against Maurice FitzGerald. Upon Gofraidh's death, subsequent to wounds incurred during the battle against Ó Néill,^{[citation needed]} he was succeeded in the chieftainship by his brother Domhnall Óg, who returned from Scotland in time to withstand successfully the demands of Ó Néill. Over time, the O'Donnell King of Tyrconnell became known as the Fisher-King, on the Continent, ostensibly due to the export of fish traded for wine in La Rochelle.

The O'Donnells were patrons of the arts, literature, and of religious benefices. In particular, one, Manus, wrote the biography of ColmCille (St. Columba). They also were the patrons of the Franciscans in Donegal Abbey. They also exercised "jus patronus" to nominate bishops.

In the early 14th century A.D., the O'Donnell rulers aided Templar knights fleeing via Sligo and Tyrconnell to Scotland where a Templar priory existed at Ballymote [2], a Percival family estate for the last 300 years. The Stewarts relied on the O'Donnells in the balance of power with the MacDonalds beginning in the fourteenth century.