1‭ April 1129 AD: The death of Cellach mac Aeda [aka Celsus] on this day. This famed and holy man was the bishop and abbot of Armagh. He was a member of the Clann Sínaig family, which had held the abbacy of Armagh since c.965. His status as head of the Irish Church had been recognised at the Synod of Rathbressail/Ráth Breasail  in the year 1111. It marked an important step in the transition of the Irish Church from a monastic to a diocesan and parish based church. Many Irish present day dioceses trace their boundaries to decisions made at the synod. Cellach was born in the year 1080 and was an advocate of reform in the Irish Church to put it on a more formal footing and along set lines that matched similar moves on the Continent.

In 1106 Cellach was consecrated as ‘noble bishop’ (uasal epscop) in Munster ‘by command of the men of Ireland’, following formal circuits of Tir nÉogain [in Ulster] and Munster as coarb (successor) of Patrick he received his ‘full due’ of tribute. He subsequently revisited Munster in 1120. He also visited Connacht (1108, 1116) and Meath (1110). The implication is that his appointment as bishop of Armagh gave him a position of pre-eminence in the Irish church, before the actual establishment of a diocesan hierarchy in 1111 at Rathbressail.

Apart from his normal administrative duties, Cellach, as coarb of Patrick, played a peace-keeping role on a number of occasions (1107, 1109, and 1113) in the incessant dynastic wars between the southern claimant to the high-kingship, Muirchertach Ua Briain , and his northern opponent, Domnall Ua Lochlainn , and between other warring kings on subsequent occasions. 

Dib.ie

While on a visit to Munster‭ (Mumu) he took ill and died at the religious settlement of Ard Patrick near Limerick. He was buried at his own request at Lismore /‬Lios Mór Co Waterford‭. Shortly before he died he designated the future Saint Malachy as his successor. However his protégé was to be frustrated in his attempts to secure the see of Armagh by Muirchertach mac Domnall who installed himself at Armagh before Malachy could get there. The situation was only resolved in 1134 after much political and ecclesiastical power politics had been played out. ‬We see in his ‘Obituary’ in the Annals of Ulster an eulogy that sets out the kind of person a great ecclesiastic was expected to be:

Cellach,‭ successor of Patrick, a virgin and the chief bishop of western Europe, and the only head whom Irish and foreigners, lay and clergy, obeyed, having ordained bishops and priests and all kinds of cleric also, and having consecrated many churches and churchyards, having bestowed goods and valuables, having exhorted all, both laity and clergy, to uprightness and good conduct, after a life of saying the hours, saying mass, fasting, prayer, after being anointed and having made excellent repentance, sent forth his soul to the bosom of angels and archangels in Ard Pátraic in Mumu on Monday, the Kalends 1st of April, the twenty-fourth year of his abbacy and the fiftieth year of his age.

His body was brought on the third of the Nones‭  [3rd of April]  to Lis Mór of Mo-Chutu in accordance with his own testament, and was waked with psalms and hymns and canticles, and buried with honour in the burial-ground of the bishops, on the day before the nones 4th of April, that is, Thursday. Muirchertach son of Domnall was appointed to the successorship of Patrick on the fifth of Nones [5th of April.]

Annals of Ulster