He took over a kingdom that was on the rise and although never gaining the importance of his father he was still an intergral part of the political history of Eire, playing a leading role in the permanent split of the Ui Conchobhair.
In 1395 Maol Seachlainn was convinced by his uncle Muirchertach, Archbishop of Taum to submit to the rule of Elizabeth I. It was for Muirchertach that the Book of Ui Mhaine was compiled starting in 1392 when he was still Bishop of Clonfert.
A praise poem Fa urraidh labhras leac Theamhrach (Found in The Book of O'Conor Don) presents him as a national figure who should be King of Tara, is hospitable, a patron of poets and more.
Maol Seachlainn died in 1401.
Mail Seachlainn Ó Cellaigh, high-king of Ui Mhaine, ie from Cara to Grian, the most outstanding man of Western Europe, defensive tower of the Irish, the best man of his time for hospitality, alertness, and wisdom, for defending churches, for supporting foreigners, the feeble, and the exiled, a sage both faithful and reverential to the Church and to confessors, and a very charitable man to the weaklings of the Creator --- this excellent man died at the height of his rule and of his powers, after the tribulation of Penance. He had been brought before death to the monastery of Cnoc Muaidehe [Knockmoy], and he was buried there in his own stone tomb, having left his family to succeed him. [The Four Masters]
The Ó Cellaigh tomb is located in the north wall of the chancel at Abbeyknocmoy. a Cistern Abbey founded in 1190. Unfortunately it was partially dismantled for the Great Exhibition of 1851 and is not in good repair. The inscription reads: DO MULEACHLAIND O KEALLAID DO RI O MANI AGUS D'INDBUALAIND INGE(N) I CHONCHUR DORINE MATHA O COGLI IN LEABAIG SEA
[For Mael-Sechalind Ó Cellaigh king o Ui Maine, and for Fionnguala, daughter of O Conchobair, Matha O Cogli made this bed]
The illustration here depicts the tomb before it was dismantled.
Fionnguala died in 1403