 Late in 999 , the Leinstermen , historically hostile to domination by either the Uí Néill overkings or the king of Munster , allied themselves with the Norse of Dublin and revolted against Brian Boru . This provided the opportunity for Sigtrygg 's second alliance with Máel Mórda mac Murchada . Brian 's forces inflicted a crushing defeat on the united Leinster @-@ Dublin army at the Battle of Glenmama , and followed the victory with an attack on the city of Dublin . The 12th @-@ century Cogadh Gaedhil re Gallaibh gives two accounts of the occupation : Brian remained in Dublin from Christmas Day until Epiphany ( 6 January ) , or from Christmas Day until St. Brigid 's Day ( 1 February ) . The later Annals of Ulster date the Battle of Glenmama to 30 December 999 , while the Annals of Inisfallen date Brian 's capture of the city to 1 January 1000 . In any case , in 1000 Brian plundered the city , burned the Norse fortress and expelled Sigtrygg . 
