Traditional accounts ascribe different reasons for this battle. The most famous is the story about the copying of a book belonging to Saint Finnian by Columcille. According to tradition, sometime around 560, the Irish abbot and missionary Saint Columba became involved in a quarrel with Saint Finnian of MovillaAbbey over a psalter. Columba copied the manuscript at the scriptorium under Saint Finnian, intending to keep the copy. Saint Finnian disputed his right to keep the copy. Thus, this dispute was about the ownership of the copy. King Diarmait mac Cerbaill gave the judgement, "To every cow belongs her calf, therefore to every book belongs its copy." Columba disagreed with King Diarmait's ruling against him and reportedly instigated a successful rebellion of the Uí Néill clan against the King. The battle was claimed to have caused around 3,000 casualties. Another traditional reason given for the battle concerns the violation of sanctuary. The Annals Of Tighernach for the year 559, record the death of Curnan son of Aed, son of Eochaid Tirmcharna by Diarmait mac Cerbaill, while Curnan was 'ar comairce Coluim chilli - under Colum Cille's protection'. This Curnan was the son of Aed, the king of Connacht. That Diarmait had violated the sanctuary of the termonn at the Columban monastery of Kells is given as the reason for the battle. Either way, as a monk who had taken up arms, Colmcille faced judgement for his deeds. It said that the judgement resulted in Columba leaving his homeland for Scotland, where he founded Iona Abbey in 563. The sentence stipulated he was to win as many souls to Christianity as had been lost in the battle. The Chronicles of Iona state that "In the second year after the battle of Cul-drebene, the forty-second year of his age, Columba sailed away from Ireland to Britain wishing to be a pilgrim for Christ."
Modern investigations
However, these later versions have been disputed by modern scholarship on several grounds, particularly as early mentions of the battle do not refer to the book incident. It may have been a dynastic battle between the Cinel Cairbre and Diarmait Mac cerbaill who had wrested the kingship at Tara from them after the death of Tuathal Maelgarb.
