This ancient Irish surname has two possible origins. Firstly, it may be an Anglicized form of the Gaelic personal name "Cillin", a diminutive of "Ceallach" which means "bright-headed". The name was borne by various early Irish saints, including the leader of a 7th Century mission to Franconia and Thuringia, hence the popularity of the given name Kilian in medieval central Europe, from which several surnames are derived. The second possibility is as an Anglicization of the Gaelic "Mac Coilin" which is composed of the elements "mac" meaning son of, with the personal name Colin, which derives from the Celtic "Cailean" meaning a young hound. in the modern idiom the surname has several including Killeen and Killen. A gallowglass family brought from Scotland by the O' Donnells to fight on their behalf introduced the name to Ireland and it is now found mainly in Oriel, a territory comprising Counties Armagh, Monaghan and parts of South Down, Louth and Fermanagh. Two Belfast men, Rev. Thomas Young Killen (1826-1886), and Rev. William Dore Killen (1806-1902), were notable as leading Presbyterians; also from Co. Antrim was James Bryce Killen (1845-1916), the Fenian and co-founder with Michael Davitt of the land league. The first recorded spelling of the family name is shown to be that of Cormac O' Killeen, which was dated 964 A.D., Bishop of Clonmacnois, in Ancient Irish Records, during the reign of King Brian Boru, High King of Ireland, 940-1014. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.