In Irish Ó Duinn or Ó Doinn it is more often written Dunne than Dunn in English. The form O’Doyne, common in the seventeenth century, is now almost obsolete. In fact, of 364 births registered for them in a given year, 313 had the final E and only 51 were Dunn. From this it can be estimated that the total number of people so called in Ireland today is approximately 15,000, giving them twenty-seventh place in the list of commonest surnames in Ireland. Conventional wisdom suggests that the name originates from the Irish word “donn” meaning “brown”, “doinn” being the genitive case. However, there may be some truth in the earlier assertion that it is from “dun” meaning “hill”. This would tie in nicely with the family war cry and motto “mullach abú” – “the (people of the) hill forever”.

This sept originated in Co. Laois (Queen’s County) and formed one of the principal families of Leinster, their chief being lords of Iregan (from Uí Riagain – descendants of Regan) in that county. Their ancient territory is in the current barony of Tinnahinch, with Brittas being a major stronghold. One ancient record refers to them in the following terms “Over Uí Riagain of the mightily victories are active warriors who conquer in battle, O Dunn is the chief of the conquering troops, the mainstay of the battling spears”. The sept is one of those specially mentioned in the mid-sixteenth century official orders as hostile and dangerous to the English interest.

It is in Laois that Dunnes are, appropriately, now to be found in greatest numbers, though they have spread far and wide. In 1659 Dunn is found as a principal name in Dublin, Kildare and Offaly with McDunn in Fermanagh. Dun is found in Offaly and Laosi at that time. In 1890 Dunn is again found in Ulster, with Dunne in Laois, Offaly, Dublin, Kilkenny, Kildare, Cork, Tipperary and Cavan. Nearly all those who spell the name Dunn came from Ulster. This is a name to which the practice during the present century of resuming the discarded prefixes Mac and O does not apply – the form O’Dunn or O’Dunne is seldom if ever seen today. The Dunns and MacDunns of Ulster may be of Scottish rather than Irish origin.

At least one of the name is to be found in the gallery of famous Irishmen – Gillananaomh Ó Duinn (1102 – 1160), the historian and poet.

Teige O’Doyne, of Castlebrack, Queen’s County, prince of Iregon, and chief of his name; was living in 1593; had five sons, and a brother named Tirlogh, who was the ancestor of another branch – Dunn of Ards. It is noted that the Castlebrack tenants of this Teige O’Doyne paid one penny “heriot” per acre, on the death of each Ceannfinne or chief head of a family. The word heriot means “a fine paid to the lord of the manor at the death of a landholder.” His tenants of Kernymore paid yearly – two beeves, twenty-four crannochs of oats, forty cakes of bread, thirteen dishes of butter, seventeen cans of malt; eight pence, heriot, in money, on the death of each Ceannfinne; one reaping hook (service) on one of every twenty acres; custom ploughs one day in winter and one in summer. From inhabitants of Ballykeneine Quarter: Meat and drink for twenty-four horse boys, or four shillings for their diet. From the inhabitants of Cappabrogan: like duties. From Garrough: like duties. These “Chief Rents” were A.D. 1613, abolished in Ireland in the reign of King James the First, by the Parliament then held in Dublin by the Lord Deputy Sir Arthur Chichester

One member of the sept was killed at the battle of Aughrim in 1691. Another very active Jacobite was James O’Dunne (circa 1700-1758), Bishop of Ossory, most of whose life was spent in France, in the service of which country several of his relatives distinguished themselves as diplomats and soldiers.

In modern times Charles Dunn (1799 – 1872), was a notable judge in the U.S.A. and Colonel Humphrey O’Dunne was famous for his bravery in the attack on Savannah in 1774.

The Irish-American author Finlay Peter Dunn, created the humorous character “Mr. Dooley”, an Irish-American saloon keeper in Chicago. His Mr. Dooley books were published between 1898 and 1919.

Sir Patrick Dun (1642 – 1713), five times President of the Royal College of Physicians, Ireland and Irish M.P., whose memory is perpetuated in Sir Patrick Dun’s Hospital, Dublin, was of a Scottish family.”

Source: Irish Families, Edward MacLysaght, Irish Academic Press, 1991