Like so many well known Irish surnames, Donaghy, MacDonagh, MacDonnchadha are formed from a common christian or personal name.

MacDonagh is one which came into being independently in two widely separated parts of the country. We usually find the modern descendants of their original ancestors in considerable numbers in the territory of their origin (or, where dispossessed by invasion, in the territory of their subsequent settlement). MacDonagh, however, would appear at first sight to be an exception to that general rule.

The MacDonagh sept whose chiefs' in County Cork held the strong castle of Kanturk and were known as Lords of Duhallow, was a branch of the MacCarthys. The name MacDoangh is now rare there, but there is reason to believe that these MacDonaghs in many cases assumed the surname MacCarthy.

Taking the last year for which statistics are available we find that of 174 MacDonagh births registered in the period only eleven were in Munster and nine in Ulster, while as many as 141 were in Connacht. As Leinster includes Dublin the 13 recorded for that province may well be children of Connacht residents in the capital.

It is clear from this that the other MacDonagh sept has survived vigorously in the province of its origin. These MacDonaghs were a branch of the great MacDermot clan, whose chiefs long held sway in Counties Sligo and Roscommon; the MacDonagh chiefs were lords of Corran or Tireril in County Sligo. In this note the name has been spelled MacDonagh throughout, but in fact only 100 of the 174 births referred to above were so entered, the other 74 being given as MacDonogh, MacDonough etc.

Though lack of uniformity in the spelling of names has little significance prior to the introduction of compulsory registration in 1864, it is noticeable that three individuals who may be cited as noteworthy Irishmen of the name each used a different spelling.

John MacDonogh (1779-1850), the Irish-born American philanthropist, is best remembered for his efforts on behalf of slaves;

Thomas MacDonough (1783-1825) another Irish-American, was a naval officer who greatly distinguished himself at he battle of Plattsburg;

Thomas MacDonagh (1878-1916), was not only a poet of distinction but also a leader of the Rising of Easter 1916 and a signatory of the Declaration of Independence, being one of those who was a victim of the long drawn out executions which followed that event.

Andrew MacDonagh (b. 1738), who accompanied Wolfe Tone in the Bantry Bay expedition in 1796, was of a Sligo family which was able to boast that forty of its members served in France in the Irish Brigade.