The O'Grady sept originated in Co. Clare and may be classed as Dalcassian, though the seat and territory of the Chief of the Name has for several centuries been at Killballyowen, Co. Limerick. The present holder of that dignity (i.e. in popular parlance "The O'Grady") is one of the very few the authenticity of whose claim to chieftainship is officially recognized in Ireland.

The name in Irish is O Gradaigh or more shortly O Grada, so that the anglicized form approximates closely to the original. A peculiarity about it is that its leading family in Co. Clare, who favoured the English invaders in the time of Henry VIII, gradually changed their name from O'Grady to Brady, being described in legal documents of the sixteenth century as "O'Grady alias Brady" or vice versa.

Thus the Bradys around Tuamgraney in East Clare are really O'Gradys, though Brady is itself a common name in Ireland (especially in north Leinster and south Ulster) having no affinity with O'Grady at all. The ancestor of the present chief, though known at the time of his migration in Co. Limerick as John O'Grady alias Brady, dropped the latter and his descendants have ever since used the ancient and correct form of their name.

If we examine the distribution of the name in modern times we find that, combining the separate returns for O'Grady and Grady (which are of course the same name), the total is not inconsiderable amounting to some four thousand all told. The majority of these hail from Co. Clare as might be expected. This is followed by Mayo which is of interest because it has been stated, on what authority I cannot say, that there was a distinct O'Grady sept originating in Mayo - more probably it was an offshoot of the Dalcassian stock.

Many of the Gradys of Mayo and Roscommon are Greadys - Mag Riada in Irish, corrupted in the spoken language to O Griada. In Co. Tipperary Grady is often Gready in Disguise, the Irish form there being O Greada. This, no doubt, is the original of the names Gredy and Graddy which were regarded as numerous in the barony of Middlethird (Co. Tipperary) in 1659.

There is a constant tendency for uncommon names to be assimilated to better known ones of a similar sound: thus Gready tends to become Grady, as does Graddy in Kerry. John O'Grady was Archbishop of Tuam from 1364 to 1372.

In modern times several members of the Co. Limerick O'Gradys have distinguished themselves in the service of Britain, one Standish O'Grady (1768-1840) being created Viscount Guillamore. The forename Standish with O'Grady is perpetuated by Standish Hayes O'Grady (1832-1915), who has been called "the last of the grand old scholars of Ireland".