The Lead Up to 1002
Tensions between the high-king Máel Sechnaill and Brian Boru increased after Brian's victory over the 'Foreigners of Dublin' in 1000. Soon after this battle, Brian challenged the high-king directly by marching on Tara, but as the Annals of Ulster record, Brian was the one who ended up retreating without a fight:
Brian made a hosting to Ferta Nime in Mag Breg. The foreigners and the Laigin, with a raiding party of horsemen, came before them into Mag Breg, and Mael Sechnaill came upon them, and they were nearly all killed. Brian then retreated without giving battle or making incursion—by the Lord's insistence."
The Annals of Ulster are biased in favour of Máel Sechnaill. Unfortunately, there is no surviving accounting of this battle in the Annals of Inisfallen, which were compiled in Munster, and often offer a pro-Brian description of events. What exactly happened on that day and why Brian retreated remains, therefore, a mystery.
For the following year we learn from contemporary sources that Brian's forces grew stronger. Munster, Leinster and the Norse of Ireland became part of Brian's army and the Annals of Inisfallen record a joint expedition into Connacht.
Brian becomes High-King
In 1002, Brian undertook a hosting in Athlone that was also attended by Máel Sechnaill. At that meeting, Máel Sechnaill offered Brian the hostages of Connacht and Mide. It is also possible, that it was here in Athlone that Brian gained the title of high-king from Máel Sechnaill. The Annals of Ulster only record the hosting:
Brian brought an army to Áth Luain and took the hostages of the Connachta and of the men of Mide."
If it was at this point that Máel Sechnaill recognised Brian Boru as the new high-king of Ireland it meant that, for the first time in a long while, Ireland supposedly had one overlord. Yet, it is clear that Máel Sechnaill, though he had been king of Leth Cuinn (the northern half of Ireland), did not have effective control over a number of population groups in the north.
The northern Uí Néill dynasties of the Cenél nÉogain and Cenél Conaill were particularly problematic.
Therefore, even though Máel Sechnaill submitted to him, in order to become high-king of Ireland (ard-rí Érenn) in anything other than name alone, Brian was forced to complete a number of circuits throughout the north taking hostages and enforcing his overlordship. Brian started his campaign to dominate the north later that same year by leading an army to Dún Delca (Dundalk) to demand hostages from the Ulaid. The Annals of Ulster mention Brian and Máel Sechnaill leading the army together, but the Annals of Inisfallen omit any mention of Máel Sechnaill's participation. This omission might be understood if we remember that by the time Brian attacked Dún Delca, he had already been given the power of high-kingship by Máel Sechnaill, and if the Annals of Ulster are to be trusted and Máel Sechnaill was in Dún Delca, it was as Brian's vassal, not his equal.
Dún Delca was specifically chosen because it was a place associated with the Gaelic hero Cú Chulainn and guarded the entrance into the kingdom of the Ulaid. Brian met with Dál Fiatach, king of Ulaid, Eochaid mac Ardgail, and Áed Ua Néill of Ailech. No one is quite sure if the meeting between Brian and the Ulaid in Dún Delca was the meeting of two armies fighting for political control or if it was instead a conference between two kings, who only brought their armies for support. Whatever actually happened, the records agree that after this meeting they came to some form of agreement: Brian parted in peace, but without the Ulaid's hostages. (Obtaining hostages from a king meant that they had submitted to your overlordship. The fact that Brian left without hostages means that the Ulaid were still a problem for his control of the country.)
In 1005 and 1006 Brian returned to the north and visited Armagh (1005), the religious centre of Ireland. On this trip, the annals record that he finally managed to obtain hostages from the Ulaid.
Seán Duffy, Brian Boru and the Battle of Clontarf (Dublin, 2013).
Ailbhe MacShamhráin, 'Brian Bóruma, Armagh and High Kingship' in Seanchas Ardmhacha: Journal of the Armagh Diocesan Historical Society, vol. 20, no. 2 (2005), pp 1-21.