In December of the year 999, after celebrating Christmas on a march to Dublin, the Munster king Brian Boru won a major battle at Glen Máma near Dublin after being intercepted by the Norse of Dublin and the Leinstermen on Thursday 30 December.

• ​Only the Clonmacnois set of annals, pro-Úi Néill, mentions Máel Sechnaill (king of the Southern Uí Néill) as taking part in the expedition on Brian’s side.

Where exactly this battle took place is still under debate in modern scholarship. Brian’s route, coming from Munster, is, unfortunately, unknown. Suggestions for the location of Glen Máma have included: Dunlavin in County Wicklow, Newcastle Lyons, in County Dublin (possibly even the Glen of Saggart), or a gap (now crossed by the Naas Road) between Kill and Rathcoole. In any event, it is likely that the battle took place within a days march of Dublin as Brian continued on to Dublin in the immediate aftermath of the battle.

According to the Annals of Inisfallen, Brian attacked and defeated the ‘foreigners of Áth Cliath’, including the Norse ruler of Dublin, Sitric Silkenbeard, and his allies the Leinstermen, including Máel Mórda, king of Leinster (uncle of Sitric Silkenbeard)

There were many casualties on both sides. According to the Cogadh Gáedhel re Gallaibh the battle was:

bloody, furious, red, valiant, heroic, manly; rough, cruel, heartless; and men of intelligence and learning say that since the battle of Magh Rath, to that time, there had not taken place a greater slaughter.”

Todd, Cogadh Gaedhel re Gallaibh, (London, 1867), p. 111.

Amongst the Dublin army, Harald son of Amlaíb (Sitric Silkenbeard’s brother), and Cuilén son of Eitigén (who belonged to the Gailenga of Meath) were killed.

As for Brian’s side, most texts note that there were casualties, but no details are provided. Only the Cogadh notes that:

there fell multitudes of the Dál Cais and of the Muimhnigh (Munstermen) in general.”

Todd, Cogadh Gaedhel re Gallaibh, (London, 1867), p. 111.

According to Ailbhe MacShamhráin, this battle may have given Brian the ‘psychological advantage’ over Máel Sechnaill and ‘increased his readiness’ to break the agreement brokered in 997, bringing Brian one step closer to Clontarf.