On the death of the previous chief, his grandfather Sithric ‘Carrach-in-Cairn’ Mág Tighearnán in 1290, Matha took the chieftaincy and resided in the castle of Croaghan of the Cups (Irish- Cruachan O'Cúbhrán), now in the townland of Coolnashinny, besides the modern town of Killeshandra.
The Annals of Loch Cé for 1293 state- Ferghal O'Raighilligh, chieftain of Muinter-Maelmórdha during the space of twelve years, was killed in the middle of his own residence by the Tellach-Dunchadha, in hoc anno.
Poem 9 in the Book of Magauran[3][4] is dedicated to Mrs Gormlaidh O'Reilly-McGovern and was written at the time of Matthew O'Reilly's death by the poet Maol Pádraig Mac Naimhin (or Cnáimhín).
Stanza 37 refers to Matthew's death as follows- Matha's gracefulness could be seen beneath his fair hair even as he was laid low by the hands of his foes; Ó Raighilligh ever near to spears ruins Magh Fáil by his grave.
Matha son of Giolla Íosa Ó Raighilligh, king of Bréifne, was killed, together with gallowglasses, at Achadh na Corra by the Teallach Dúnchadha.
The Annals of Connacht for the year 1314 state- Matha Mag Tigernain, chieftain of Tullyhuncoe, was killed in treachery by Cathal son of Domnall O Ruairc, his foster-son, on the floor of his own house.
The Annals of Loch Cé for the year 1314 state- Matthew Mac Tighernain, dux of Tellach-Dunchadha, was slain by Cathal, son of Domhnall O'Ruairc, i.e. his foster-son, in the middle of his own house, by treachery.