Sometime during the years when Brian was King of Munster, a royal wedding was to be held near Limerick. The King of Leinster, allied with the Limerick Vikings, was invited to attend, and, with a small contingent of his army set out to cross northern County Tipperary about 30 miles of Munster—enemy territory—to reach Limerick and the wedding. The route selected would avoid towns as much as possible, to avoid detection and confrontation with Brian's Munstermen. It was to cross the highest of the Arra Mountains, Tountinna, 1,500 feet high, where there were some old slate mines and a few farms, but no villages until reaching the River Shannon at Ballina, not far from the Limerick border.
Brian Boru's castle was atop the hilly town of Killaloe just across the Shannon from Ballina. The view from the castle looked across the river toward Ballina and the Arra Mountains. Gormlaith, bride of Brian, was at home in Killaloe when she received word of the wedding guests underway from Leinster. It so happened that Gormlaith was none other than mother of Sitric Silkenbeard, Viking King of Dublin, mortal enemy of Brian Boru and the Irish of Munster. Although Brian was at that moment away from Killaloe, Gormlaith knew an opportunity when she saw one, and proved to be no shrinking violet. Calling on her loyal friends in Dublin, Gormlaith ferreted the travel plans of the King of Leinster and his militia and planned a surprise welcome for them when they neared the end of their journey.
As the tired wedding guests traversed the heights of Tountinna and came into sight of Lough Derg, the great lake of the Shannon, and the Slieve Bernagh mountains to the west, they were set upon by the murderous attack of a superior force led by a fierce woman. No mercy was shown. The entire wedding party—including the King of Leinster—was slain on the slopes of Tountinna. They were buried on the spot, and the graves marked with several medium sized blocks of native stone.