The O'Tooles of Leinster, one of the leading families of that province, are descended from Tuathal Mac Augaire, King of Leinster (died 958), who belonged to the Uí Dúnlainge dynasty. The name is an anglicization of the Irish Ó Tuathail.
Their original territory comprised the southern part of the present County Kildare but they were driven from it during the Anglo Norman invasion and settled in the mountains of what is now County Wicklow around Glendalough.
Here, with their kinsmen the O'Byrne family, they were noted for their resistance to English domination for four centuries.
At the start of the 16th century, there were five great houses, all, owing allegiance to "The O'Toole of Powerscourt" as the recognized chief:
- O'Toole of Castle Ruddery, residing in Glen Imaile.
- O'Toole of O'Toole's Castle, Ballymacledy, (now Talbotstown), Glen Imaile.
- O'Toole of Carnew Castle.
- Art Oge O'Toole of Castle Kevin, Fertie.
- Tirlogh O'Toole of Powerscourt, Feracualan.
- O'Toole of Omey, Iar Connaught, with other minor houses of the family such as OToole of Ballineddan and Brittas, in the Glen Imaile; O'Toole of Toolestown, near Dunlavin; O'Toole of Glengap, or Glen of the Downs (as it is now called); and a few others.
At the start of the 16th century, the leading branches of the clan were to a certain extent independent of each other; they were all bound to protect themselves; but in external matters affecting the whole clan they were bound to obey the head of the sept.
Throughout their history the family were famous as soldiers, from fighting the English in the glens of Wicklow to serving in the armies of other European countries in the 18th century.
Notable members 
- Saint Laurence O'Toole was an archbishop of Glendalough and in 1171, while he was Archbishop of Dublin he took up arms against the Anglo Norman invaders.'
- Mor O'Toole, 1st queen of Dermot MacMurrough and mother of Aoife of Leinster
- Colonel John O'Toole of the Irish Brigade in France was created a count and is the ancestor of the present Count O'Toole of Limoges.
- Sergeant O'Toole of the 9th Queens Royal Lancers of the British Army was awarded a Victoria Cross for conspicuous bravery in the Zulu War on 3 July 1879.
- Peter O'Toole is a notable Irish stage and screen actor who starred in such films as Lawrence of Arabia, The Lion in Winter, and Goodbye, Mr. Chips.
- Fintan O'Toole is an award winning Irish journalist, author and critic, best known for his work with The Irish Times.
See also 
- O'Toole, Patrick Laurance (1890). History of the Clan O'Tool and other Leinster Septs. Dublin: M. H. Gill and son; New York, Benziger brothers; [etc., etc.]
This article incorporates text from a publication now in the public domain: "History of the Clan O'Tool and other Leinster Septs" by Patrick Laurance O'Toole