 In the same year, Brian and Máel Sechnaill began co-operating against the Norse of Dublin for the first time.Late in 999, however, 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.The "Annals of the Four Masters" records that Brian and Máel Sechnaill united their forces, and according to the "Annals of Ulster", they met the Leinster-Dublin army at Glenmama on Thursday, 30 December, 999.