MacDonald is extremely numerous and widespread throughout Ireland. It is commonly a confusion for MacDonnell (q.v.), and shares the same origin, coming from the Gaelic personal name Domhnall, meaning "world mighty". However, true MacDonalds are descendants of the Scottish clan of the name. They are one of the group of Scottish clans who claim descent from Conn of the Hundred Battles, the legendary Irish king, through Colla Uais, who colonised the Hebrides. Their name comes from Donald of Islay, one of the sons of Somhairle, Lord of Argyle. By the fifteenth century, they were the most powerful clan in Scotland, controlling the entire western coast of the country. Their involvement in Ireland was continuous from the thirteenth century, when they first arrived as gallowglasses, or mercenaries; such was their fame that they were employed in virtually every local war, spreading and settling throughout the country over the following centuries. Inevitably, their main connection remained with Ulster. A secondary influx into that province of settlers bearing the name occurred in the eighteenth century, when the Highland clearances caused great forced migration from Scotland.
MacDonnell comes from the Irish Mac Domhnaill, from the personal name Domhnall, a compound made up of "world" and "strong". It is common and widely distributed throughout Ireland. The principal source of the name in Ireland is in fact Scottish. Grandsons of Donald of Islay came to Ulster as gallowglasses, mercenary fighters for the Irish chiefs, in the 14th century. They rapidly acquired territory and power on their own account, both in Ulster - branches of the family established themselves in Tyrone, Fermanagh, Monaghan and Antrim - and elsewhere - the Burkes are reputed to have first brought them to Mayo. In the sixteenth century Somhairle Buidhe ("Sorley Boy") MacDonnell conquered a large part of Co. Antrim and defended it tenaciously against Gaelic Irish and English intrusions. In 1620 his son, Randal MacSorley MacDonnell was created Earl of Antrim. Given the origin of the surname, the confusion with MacDonald is understandable; Irish MacDonalds in fact share their ancestry with most Irish McDonnells. In addition, the Antrim pronunciation of the original Irish has sometimes lead to the anglicisation MacConnell, or Connell.
To complicate matters still further, there are also several native Irish sources for the surname. In Ulster, the most prominent native family were the MacDonnells of Clankelly, rulers of Fermanagh before the rise of the Maguires. Displaced by their loss of power, they settled in the north of the adjoining Co. Monaghan, and remain numerous in the area. Another family arose in the old kingdom of Thomond, in the Clare/Limerick area, where the MacDonnells were hereditary poets to the O'Briens.
Sean Clarach MacDonnell (1691-1754) was the most famous poet in Munster in his day.
.Alexander MacDonnell (1798-1835), from Belfast, was one of the greatest chess players of his day. He was world chess champion in 1833.
Sir Antony MacDonnell (1844-1925) of the Mayo family had a distinguished career in the British civil service, becoming Lieutenant-Governor of Agra and Oudh in 1895. From 1902 to 1908 he was the head of the Irish civil service and played a large part in the reforms of his day, including the transfers of land ownership to small tenants. He was made Baron MacDonnell of Swineford in 1908.