Battle of Glenmama The Battle of Glenn Máma ( , The Battle of " The Glen of the Gap " ) or Glenmama was a battle that took place in County Wicklow in AD 999 . 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 . The two armies met in a narrow valley in the Wicklow Mountains , causing a rout of Máel Mórda ' s army in at least three directions . They were pursued , and the main body of the army was slaughtered when they rallied at several fording points along the River Liffey . The main commanders were either killed or captured . The battle resulted in the occupation of Dublin by Brian ' s Munster forces , and the submission of Máel Mórda and King Sigtrygg Silkbeard of Dublin to Brian Boru . The solution did not prove permanent , however , and eventually resulted in the second Leinster revolt against Brian and the Battle of Clontarf in 1014 . Sources . The battle appears as an entry in a number of the Irish annals : namely , " Annals of the Four Masters " , the " Annals of Ulster " , the " Chronicon Scotorum " and the " Annals of Innisfallen " . The Irish annals " constitute a substantial and unique collection of annual records of ecclesiastical and political events " , as written in the Irish monasteries from the mid - 6th century to the end of the 16th century . Although the historical status of the retrospective entries on the pre - Christian and early Christian periods are uncertain , entries from the later 6th century on are contemporaneous . Collation of the annals has provided a reliable chronology for events in medieval Ireland . There was cross - over between many of the annals , parts of which were copied from each other , but each collection reflects something of the monastery and district in which it was compiled . The " Annals of Ulster " reflect the viewpoint of counties Armagh , Fermanagh , Londonderry and the northern part of the province of Connacht . It was authored by Cathal Mac Manus , a 15th - century diocesan priest , and is considered one of the most important , " possibly the single most important " , record of events in medieval Ireland . The " Chronicon Scotorum " ( as with the " Annals of Tigernach " , " Clonmacnoise " and " Roscrea " ) reflects political and ecclesiastical events relevant to the monastery and environs of Clonmacnoise in Leinster . The " Annals of Innisfallen " reflect the Munster viewpoint , in particular the monastery of Emly . In the 1630s , the texts of these annals were compiled into a single , enormous compendium , known as the " Annals of the Four Masters " . In the process , the authors sometimes modified the chronology and content of some of the materials , and is thus chronologically untrustworthy . However , it is recognised that they saved for posterity material that would otherwise have been lost , and the entry contains the longest annalistic account of the battle . The battle is also mentioned in more detail in the earlier , 12th century " Cogadh Gaedhel re Gallaibh " , most recently edited by James Henthorn Todd ( 1867 ) , and includes a bardic poem commemorating the battle . " Part compilation and part romance " , it was written based on the extant annals as a propaganda work to glorify Brian Ború and the Dál gCais dynasty . More recently , its worth as a historical record has been questioned ; according to the 20th century medievalist Donnchadh Ó Corráin , it " influenced historiography , medieval and modern , out of all proportion to its true value " . However , historians still recognise it as the " most important of the Irish sagas and historical romances concerning the Vikings " . Todd includes in his translations of the " Cogadh Gaedhel re Gallaibh " a lengthy note by Rev . John Francis Shearman who was the Roman Catholic curate of the neighbourhood of Dunlavin , Co . Wicklow , one of the suggested battlesites , in the 1830s . He interpreted archaeological remains as evidence for the location . jumped to the conclusion that Dún Liamhna was another form of the name , and identified it with Dunlavin ! But the Irish form of Dunlavin is in reality Dun Luadhaion , this and Liamhain occurring in the same Irish poem in different country - sides that identify each clearly . His theory is no longer accepted and various locations for the battle have been proposed based on literary evidence . The accepted site today is the valley behind Lyons Hill and Oughterard near Ardclough Co Kildare . Background . In 997 , at a royal meeting near Clonfert , Brian Boru , King of Munster , met with his long - time rival Máel Sechnaill mac Domnaill , who was at the time High King of Ireland . Although the idea of the high - kingship is considered mainly an anachronistic invention , it came into vogue in the 10th century to denote a king who had enforced his power over external territories . Máel Sechnaill assumed the Irish high - kingship after the Battle of Tara in 980 . The two kings made a truce , by which Brian was granted rule over the southern half of Ireland , while Máel Sechnaill retained the northern half and high kingship . In honour of this arrangement , Máel Sechnaill handed over to Brian the hostages he had taken from Dublin and Leinster ; and in 998 , Brian handed over to Máel Sechnaill the hostages of Connacht . 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 . According to the 17th century " Annals of the Four Masters " , the following prophecy had predicted the Battle of Glenmama : Battle . 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 . Glenmama , near Lyons Hill in County Kildare , was the ancient stronghold of the Kings of Leinster . According to the 170 - year - old theory of Shearman , there was a valley that divided a sub - range to the Wicklow Mountains . The southern ridge faces the modern town of Dunlavin , but recent scholarship has resited the battle on the approaches to Dublin city on the Dublin - Kildare border . " It would appear that the Norse expected to reach Dunlavin , and perhaps to encamp there to meet the forces of Meath and Munster . " It seemed to Shearman that Brian had anticipated their movements , and cut off their retreat " in the narrow defile " of Glenmama . With no room for a regular encounter , the flight of the Danish army must have begun immediately . The main body of the army rallied at the ford of Lemmonstown , where thousands were said to have fallen . The remnant of the defeated army fled about a mile east of the ford to Hollywood , and were utterly routed at the ford of the Horsepass on the River Liffey . A smaller body of cavalry fled through Glanvigha , possibly to reach the ford of the Liffey at Ballymore Eustace , and some of them perished crossing the morass in Tubber . A third party fled from the valley eastward to the Bealach Dunbolg to " the shelter of the wild recesses of Hollywood and Slieve Gadoe " . Brian pursued them , and his son Murchad allegedly pulled Máel Mórda from the tree in which he was hiding . The Munster - Meath army defeated the Leinster - Dublin army ; according to the propagandist " Cogadh Gaedhel " , the battle was " bloody , furious , red , valiant , heroic , manly ; rough , cruel and heartless ; " and that there had been no greater slaughter since the 7th century Battle of Magh Rath . Later historians have also seen the battle as decisive . Ó Corráin refers to it as a " crushing defeat " of Leinster and Dublin , while " The dictionary of English history " says the battle effectively " quelled " the " desperate revolt " of Leinster and Dublin . Tradition records that " the son of the King of the Danes " , Harold Olafsson , was killed in the retreat , and was interned at the now obscure cemetery of Cryhelpe . Brian took Máel Mórda of Leinster prisoner and held him until he received hostages from the Leinstermen . It was alleged that 7000 Norse fell in the battle . This was at a time when warfare was fought on a very limited scale , and raiding armies generally had between a hundred and two hundred men . Most importantly , the defeat left the road to Dublin " free and unimpeded for the victorious legions of Brian and Maelsechlainn " . Aftermath . The victory was followed up with an attack on the city of Dublin . The 12th century " Cogadh Gaedhil re Gallaibh " gives two accounts of the occupation : that 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 " gives a date of 30 December for the Battle of Glenmama , while " Annals of Inisfallen " dates Brian ' s capture of the city two days later , to 1 January 1000 . According the much more reliable " Annals of the Four Masters " and the " Chronicon Scotorum " , Dublin was only occupied for a week by Munster forces . In any case , in 1000 Brian plundered the city , burned the Norse fortress and expelled its ruler , King Sigtrygg Silkbeard . According to the " Cogadh Gaedhil re Gallaibh " , Sigtrygg ' s flight from the city brought him north , first to the Ulaid and then to Áed of Cenél nEógain . Since Sigtrygg could find no refuge in Ireland , he eventually returned , submitted to Brian , gave hostages and was restored to Dublin . This was three months after Brian ended his occupation in February . In the meantime , Sigtrygg may have temporarily " turned pirate " and been responsible for a raid on St David ' s in Wales . Brian gave his own daughter by his first wife in marriage to Sigtrygg . Brian in turn took as his second wife Sigtrygg ' s mother , the now thrice - married Gormflaith . The cessation of revolt was followed by over a decade of peace in Dublin while Sigtrygg ' s men served in the armies of Brian . However , Sigtrygg never forgot the insult of the Ulaid , and in 1002 he had his revenge when his soldiers served in Brian ' s campaign against the Ulaid and ravaged their lands .