26‭ ‬February‭ ‬943:‭ ‬The Vikings of Dublin got a lucky break,‭ ‬when they ambushed the heir apparent to the High King,‭ ‘‬Muirchertach of the Leather Cloaks/‭ ‬Muirchertach na Cochall Craicinn‭’ [as above] ‬and slew him on this day.

Muirchertach son of Niall,‭ ‬i.e.‭ ‬Muirchertach of the Leather Cloaks,‭ ‬king of Ailech and the Hector of the western world,‭ ‬was killed by the heathens,‭ ‬i.e.‭ ‬by Blacair son of Gothfrith,‭ ‬king of the foreigner,‭ ‬at Glas Liatháin beside Cluain Chaín,‭ ‬in Fir Rois,‭ ‬on the first feria,‭ ‬fourth of the Kalends of March‭ ‬[26‭ ‬Feb‭]
 Ard Macha was plundered by the same foreigners on the following day,‭ ‬the third of the Kalends of March‭…

ANNALS OF ULSTER

Muirchertach was the son of Niall Glundubh who had himself been killed fighting the Vikings at Dublin in‭ ‬919‭ ‬AD.‭ ‬He had fought and won many battles and in one report is mentioned as leading a naval expedition against the Norsemen of the Hebrides.‭ ‬However he suffered an embarrassing episode in‭ ‬939‭ ‬when in a surprise raid his enemies‭’ ‬ships raided his fortress of Aileach‭ (‬outside Derry‭) ‬and carried him off.‭ ‬He was forced to ransom his own release to regain his freedom.‭ ‬Muirchertach,‭ ‬under the ancient rule of the kingship of Tara alternating between the northern and southern O’Neills,‭ ‬was due to replace King Donnachadh on the latter’s demise.‭ ‬Sometimes though ambition got the better of him and he clashed with his senior colleague and at other times co-operated with him.‭  ‬Muirchertach married Donnchad's daughter Flann,‭ ‬but relations between the two were not good.‭ ‬Conflict between them is recorded in‭ ‬927,‭ ‬929,‭ ‬and‭ ‬938.

His most remarkable feat came in‭ ‬941‭ ‬when he carried out a Circuit of Ireland with a picked force of‭ ‬1,000‭ ‬men and secured pledges from all the principal kingdoms and carried away with him hostages as security.‭ ‬The Dalcassians‭ (‬Brian Boru’s people‭) ‬alone refused to submit.‭ But Muirchertach eventually handed over all his hostages to Donnachadh as a mark of respect.

But his luck ran out in‭ ‬943‭ ‬when he was taken by surprise by the Vikings of Dublin somewhere near Ardee,‭ ‬Co Louth.‭ ‬It looks like Muirchertach was attempting to fend off a raid by them that was heading north towards Armagh when he was taken off guard:

Muirchertach son of Niall,‭ ‬heir designate of Ireland,‭ ‬was killed in Áth Firdia by the foreigners of‭ 

Áth Cliath,‭ ‬and Ard Macha was plundered by the heathens.

Chronicon Scotorum