The patriarch of the clan was Nialláin m. Féicc m. Feidelmid m. Fiachrach Cassáin m. Collai Fochríth, who lived about 370 AD, and Senach would have been in about the 7th generation of descent from him.
Senach was born c. 550 in Cluain hui meicc Gricci (the meadow of the descendants of Grici) which was in the parish of Kilmore, Barony of Oneilland West and County of Armagh.
On the death of Eochu macDiarmaid, the Bishop of Armagh, in January 598, Senach was appointed as the 14th coarb in succession to Saint Patrick.
Thomas Walsh states, without giving the specific source, that "Senach is mentioned by several annalists as a learned man, and is said to have written valuable tracts on the Scriptures and the works of the Fathers, which are now lost.
The Calendars of the Saints have the following entries: "Senior, the son of Moeldavan, and Primate of Armagh" The Battle of Moira was fought in 637 AD and in an old text called "The Battle of Magh Rath", Senach appears in the following account of the wounding of Cenn Fáelad mac Ailella: Now the robust, sanguine, rapid-wounding hero, and the lively, sure-striking bear, Congal Claen, went forth, and was met by Cennfaeladh, the son of Oilell, to whom he gave a mighty, hard-smiting stroke of his sword, so that he broke the helmet and cut the head under it, so that a portion of the brain flowed out, and Cennfaeladh would have fallen by Congal on the spot, had he not been protected by Crunnmael, the son of Suibhne, and Maelodhar Macha; and after protecting him they conveyed him to Senach, Comharba of St.Patrick and returned to maintain their part of the battle.