The Ua Domnaill (O’Donnell) dynasty, were a leading family of the northern Ui Neill, and became the rulers of the lordship of Tir Conaill in the late medieval period. They came to prominence about the year 1200 c.e., when the first Ua Domnaill ruler, Eigneachan (c. 1201-1207) came to power. Previous to this, the O’Donnells were local kings of Cenel Lugdach in northern Tir Conaill, with a crannog at Lough Gartan and an inauguration site at Kilmacrennan. The Ua Domnaills dispossessed the previous ruling dynasties of Ua Mael Doraid, who may have died out around 1197, and Ua Canannain, the last lord of whom, Ruaidri, was deposed and slain in 1248. The Ua Domnaill lords proved themselves to be an innovative and talented, but very violent, family. In the mid-thirteenth century, they became the first Irish dynasty to employ galloglass mercenaries, in their case the Mac Suibne (Mac Sweeney) family, who became deeply established in three separate branches in Tir Conaill. The Ua Domnaill chieftains, Mael Sechlainn (1241-1247), Gofraid (1248-1258), and Domnall Oc (1258-1281), were prominent fighters against English colonialism in the northwest. Gofraid in particular defeated the chief governor Maurice Fitzgerald, in "a brave battle . . . in defense of his country," at Credran in Cairbre (Carbury) in 1257, a battle that succeeded in keeping the English out of Tir Conaill. The O’Donnaills also violently resisted all attempts by the Ua Neill lords of Tir nEogain to establish provincial hegemony in Ulster. However, they also fought fierce internal civil wars throughout the fourteenth century until the powerful Ua Domnaill chieftain, Tairrdelbach an Fhiona (1380-1422) established himself as lord of Tir Conaill.
The ruling Ua Domnaill dynasty enjoyed crucial support from the Ua Gallchobhair (Gallagher) family, who commanded their household troops, and Ua Baoigill of Boylagh and Ua Dochartaig, lord of Inishowen, who were Ua Domnaill’s two most important subchieftains. The ruling Ua Domnaills were also great patrons of the Gaelic learned classes, endowing their chief practitioners, such as Ua Cleirigh, ollamh in history, and Mac an Bhaird, ollamh in poetry, with much land. At the same time, the Ua Domnaills amassed great wealth through the exploitation of salmon and herring fisheries, becoming known as "king of fish" on the continent. The Ua Domnaill lords had a particularly good relationship with merchants from the city of Bristol in England, who traded wine, firearms, and luxury goods for the fish, tallow, and hides exported from Tir Conaill.
Tairrdelbach an Fhiona’s son Niall Garbh, lord of Tir Conaill from 1422 to 1439, was an innovative ruler. He joined with Eogan Ua Neill, the lord of Tir nEogain, to raid the English Pale, but was captured in 1434 and imprisoned, first in London, and then in the Isle of Man. A long civil war followed his imprisonment, fought between Niall Garbh’s sons and his brother Neachtan, lord of Tfr Conaill from 1439-1452. This war only ended in 1497 with the assassination of Neachtan’s brother, Eigneachan Mor Ua Domnaill.
From 1461 to 1555 Tfr Conaill was ruled by a series of three very successful Ua Domnaill warlords, Aed Ruad (1461-1505), Aed Dub (1505-1537) and Maghnus (1537-1555), who were father, son, and grandson. Shrewd and religious, these three rulers expanded Ua Domnaill power into the neighboring lordships of Fermanagh and Lower Connacht, and they called themselves "Prince of Ulster," in direct opposition to the claims of Ua Neill of Tfr nEogain. Aed Ruad, aided by Mael Muire Mac Suibne, seized power in Tfr Conaill in 1461. In 1481, he inflicted a severe defeat on Mac William Burke in Tirawley. A deeply religious man, Aed Ruad introduced the Franciscan Observant order into Tfr Conaill, establishing a monastery at Donegal in 1474. Aed Dub Ua Domnaill was in an unusually secure position in Tfr Conaill, so much so that he went on a two-year pilgrimage to Rome from 1510 to 1512, during which he stopped off for thirty-two weeks at the court of Henry VIII, who knighted him. The highlight of his career was his defeat of Conn Bacach Ua Neill at the battle of Knockavoe, fought near Strabane in 1522. Aed Dub’s son, Maghnus Ua Domnaill, was a canny ruler. Involved in the Geraldine League, he also made attempts to be made earl of Sligo. Maghnus was a noted Gaelic scholar, composing poetry and commissioning a biography of Colm Cille, the Betha Cholaim Cille, in 1532. All three pioneered the hiring of "redshank" mercenaries (Scottish Highland soldiers). They also utilized firearms, guns being mentioned in Tfr Conaill from 1487. These Ua Domnaill chieftains also had close links with the Stuart kings of Scotland. In 1495, Aed Ruad visited James IV when he "went to the house of the king of Scotland." In 1513, Aed Dub also visited James IV, where Ua Domnaill received a suit of clothes, £40 in plate, £160 in cash, in addition to the promise of a cannon and a culverin. Artillery arrived in Tfr Conaill in 1516 when a French knight brought over an artillery piece sent by the earl of Albany. From 1534 to 1537, Maghnus Ua Domnaill was in contact with king James V.
Following the deposition of Maghnus in 1555, his two sons, Calbach (1555-1566) and Aed (1566-1592) were weak rulers, in whose time Tfr Conaill descended into anarchy as Shane O’Neill terrorized the lordship. However, the famous Red Hugh Ua Domnaill, lord of Tfr Conaill from 1592 to 1602 reestablished Ua Domnaill power when he joined in the great Gaelic confederacy, which fought the Nine Years’ War. Red Hugh participated in the major Irish victory at the Yellow Ford in 1598 and won a spectacular success in his own right in the Curlew mountains in 1599. However, following defeat at the battle of Kinsale (1602), a loss for which Ua Domnaill was blamed, Red Hugh left for Spain where he subsequently died. His brother, Rury, was created earl of Tfr Conaill by King James I, but fled Ireland in the Flight of the Earls in 1607. Rury died in Rome in 1608, thus ending Ua Domnaill power.