From Manx Soc vol IV,VII & IX
Magnus, after his descent on Wales, sailed for Ireland, which he invaded and ravaged with a powerful army. A pitched battle was fought in consequence, on the plains of Magh Coba, near Downpatrick, in which his army was defeated, and he himself slain. According to the Icelandic Sagas, he was treacherously led into an ambush by the Irish, when peaceably awaiting a supply of cattle for his troops, and so lost his life. The following is the account :
" The eve of St. Bartholomews day was the time appointed by the Irish for the supply of the cattle ; but not having kept their promise, Magnus landed with his troops on the ensuing morning. The day was serene and beautiful, and the sun shining brilliantly in the heavens. He made his way over bogs and marshes, till he reached an eminence from which an extensive prospect could be had of the country. Here he saw at a distance a great cloud of dust, such as produced by a body of horsemen. Some said they were troops advancing to attack them ; others that they were men with the cattle, which, on a nearer approach, turned out to be the case. It being now noon, Magnus with his attendants returned towards their ships with the oxen. When they reached the marshes, an ambuscade in an adjoining wood suddenly rushed forth and fiercely attacked them. Magnus ordered Eyvinder, his second in command, to sound the trumpet and summon his men to the royal standard, at the same time ordering them to close their ranks with serried shields, until they crossed the swamps and reached the level ground, when they would be safe. They made their way as far as an old en-trenchment, but being closely pressed by the Irish, fell in great numbers. The king seeing a certain chief of the name of Thorgrim, belonging to Upland in Sweden, called to him saying, Do you with your cohort cross the rampart, whilst we defend you, and when over, occupy the opposite hill with your archers till we join you. Thorgrim and his men did as they were ordered, but directly they had passed the ditch, placed their shields on their backs, and fled to the ships. Magnus, seeing them fly, exclaimed, Is it thus you run, you coward ! I was a fool to send you instead of Sigurd, who would not thus desert me ! The king fought with incredible valor, but whilst defending himself against great numbers was transfixed through the thighs by a spear. Pulling it out, he snapped it beneath his feet exclaiming, Thus we young warriors break these twigs,fight on bravely my men, and fear no danger for me ! Soon after, however, Magnus was struck on the neck by a battle-axe and fell. Vidkunner, the son of John, killed the man who gave the fatal blow, and carried off with him the kings sword and the royal standard. An immense number of Norsemen were slain in this engagement, together with Swerker, Eyvinder, Tllfr, Dago, and many other chiefs. The remainder, with Magnuss son Sigurd, earl of Orkney, fled to their ships and departed."