Cellach of Armagh
Cellach of Armagh or Celsus or Celestinus (1080–1129) was Archbishop of Armagh and an important contributor to the reform of the Irish church in the twelfth century. He is venerated in the Roman Catholic Church as Saint Cellach. Though a member of the laicized ecclesiastical dynasty of Clann Sínaig, he took holy vows and gained priestly ordination. This put an end to the anomalous state of affairs, in effect since 966, whereby the supreme head of the Irish Church had been a layman. Following the Synod of Ráith Bressail, in which a diocesan structure for Ireland was established, he became the first metropolitan primate of all Ireland.