The raid on the prehistoric tombs of the Boyne valley (entered into the Annals of the four Masters at 863).
Amlaibh, Imhar and Auisle (Audgisl) three chieftains of the gaill; and Lorcan, son of Cathal, King of Meath, plundered the land of Flann (North Brega).
The viking raids on the great megalithic tombs of the Boyne valley in 863; by Olaf (Amlaibh) Ivar (Imhar) and Audgisl, probably carried out because after all the monastic raids that had been undertaken over the previous years, ‘treasure’ was by now getting hard to find. This was a great crime though, the tombs were considered sacrosanct in celtic folk-soul, so this outrage by the gaill, ( the foreigners, or in this case the Danes and Norse war lords who established themselves in Dublin).
The cave of Achadh-Aldai (Newgrange); the cave of Cnoghba (Knowth); the cave of the grave of Bodan over Dubadh (Dowth); and the cave of the wife of Gobhan at Drochat-atha (Drogheda) were broken and plundered by these same gaill.
Ivar was probably the son of the famous Ragnar Lothbrok, who left his mark at the Maes Howe tomb, Ragnar had been marooned in the tomb for three days whilst a storm raged outside. In a poetic hand scribed on the stone wall was the following;
This mound was raised before Ragnar Lothbrok’s...
His sons were brave, smooth-hide men though they were...
It was long ago that a great treasure was hidden here...
Happy is he that might find the great treasure...
So these northern pirates, who were by now becoming established on the shores of Ireland, having pillaged the monastic houses and churches of the celtic church, turned there greed for gold onto the megalithic monuments in the Boyne valley, not that they would have found much ..
the above information taken from; The Fury of the Northmen by John Marsden
The caves of Achad Aldai, and of Cnodba, and of Boadán's Mound above Dubad, and of Óengoba's wife, were searched by the foreigners—something which had never been done before. This was the occasion when three kings of the foreigners, i.e. Amlaíb and Ímar and Auisle, plundered the land of Flann son of Conaing; and Lorcán son of Cathal, king of Mide, was with them in this.
Annals of Ulster ....http://www.ucc.ie/celt/published/T100001A/index.html
Notes; "A/S Chronicle; 865 records micel haethen here, in East Anglia, Symeons of Durham describes an immense fleet of Danes, Frisians and other nations; coalition of the warbands of Ivar,Halfdan and Ubba, sons of Ragnar Lothbrok, with their lesser allies (jarls).
An immense fleet, under their kings and leaders, Halfdene, Inguar, Hubba, Baegsag, Guthrum, Oscytell, Amund, Sidroc and another leader of the same name, Osbern, Frana and Harold..
First among the warlords (p.139) stood 'the tyrant Inguar' who was to lead the host on 5 years of campaigning through Northumbria, Mercia and East Anglia..Inguar disappears from the English sources after his murder of the king of the East Angles in 870, at which point, the similarly named Imhar reappears in the Annals of Ulster laying siege with Olaf, king of Dublin, to Dumbarton on the Clyde. The same Imhar had disappeared from the Irish annals after the raid on the tombs of the Boyne valley in 863 just two years before Inguar appears in command of the great host which had landed in East Anglia and would soon seize York.
Marsden goes on to say that his identification of Ivar/Imhar/Inguar is a far from unanimous decision by historians to accepting the above identification but....he states that the chronology of the careers of the above, are one and the same man; Ivar Ragnarsson called 'the boneless', tomb-plunderer and slave-trader, son of the legendary Lothbrok and king of the northmen at both Dublin and York."..