Battle of Glenmama: The combined forces of Munster and Meath under king Brian Boru inflict a crushing defeat on the allied armies of Leinster and Dublin near Lyons Hill in Ireland.

Year 999 (CMXCIX) was a common year starting on Sunday of the Julian calendar.

The Battle of Glenn Máma or Glenmama took place most probably near Lyons Hill in Ardclough, County Kildare, Ireland, in AD 999 between Windmill Hill and Blackchurch. It was the decisive and only engagement of the brief Leinster revolt of 999–1000 against the King of Munster, Brian Boru. In it, the combined forces of the Kingdoms of Munster and Meath, under King Brian Boru and the High King of Ireland, Máel Sechnaill II, inflicted a crushing defeat on the allied armies of Leinster and Dublin, led by King Máel Mórda of Leinster.

Munster is the largest of the four provinces of Ireland, located in the south west of the island. In early Ireland, the Kingdom of Munster was one of the kingdoms of Gaelic Ireland ruled by a "king of over-kings". Following the Norman invasion of Ireland, the ancient kingdoms were shired into counties for administrative and judicial purposes. In later centuries, local government legislation has seen further sub-division of the historic counties.

Meath was a kingdom in Ireland from the 1st to the 12th century AD. Its name means "middle," denoting its location in the middle of the island.

Brian Boru was the High King of Ireland from 1002 to 1014. He ended the domination of the High Kingship of Ireland by the Uí Néill, and is likely responsible for ending Viking invasions of Ireland. Brian Boru is mentioned in the Annals of Inisfallen and in Chronicon Scotorum as "Brian mac Cennétig". The name Brian of Bóruma or Brian Boru was given to him posthumously. Brian built on the achievements of his father, Cennétig mac Lorcain, and especially his elder brother, Mathgamain. Brian first made himself king of Munster, then subjugated Leinster, eventually becoming High King of Ireland. He was the founder of the O'Brien dynasty, and is widely regarded as one of the most successful and unifying monarchs in medieval Ireland.