General Notes

Diarmait was a contender for the title of High King of Ireland. He was one of the most important and significant Kings in Ireland in the pre-Norman era. 

He was able to claim the title "King of Leinster" and install his son, Murchad, as king of Dublin. Thus ruler of two of the most powerful and wealthy towns on the island, he was able to make a bid for the High-Kingship. It was during a battle against the king of Mide, Conchobar Ua Maelsechalinn, that he was killed in 1072.

Diarmait's Death in the Annals of the Four Masters
"Diarmaid, son of Mael-na-mbo, King of Leinster, of the foreigners of Ath-cliath, and of Leath-Mogha-Nuadhat, was slain and beheaded in the battle of Odhbha, on Tuesday, the seventh of the Ides of February, the battle having been gained over him by Conchobhar O'Maeleachlainn, King of Meath. There were also slain many hundreds of the foreigners and Leinstermen, along with Diarmaid, in that battle. In it was killed Gillaphadraig O'Fearghaile, lord of the Fortuatha, &c. Of the death of Diarmaid was said:"




Two, seven times ten above one thousand,
From the birth of Christ is reckoned,
To this year, in which Diarmaid,
First man in Leinster, fell.
Diarmaid, of the ruddy-coloured aspect,
A king who maintained the standard of war,
Whose death brought scarcity of peace,
The loss of the heroes of Ladhrann, with their ships.
Comely youths were cut down there,
Together with the head of Claire and Cualann.
It caused in the breeze a noise not pleasant,
The loss of the King of Riada of great valour.
Until at Muillenn-Chul was slain
A brave chieftain of a strong fortress,
Until the furious fire-brand fell by treachery,
They found no hero who dared with him contend.
It is a red wound through my firm heart;
For the host from Caindruim it was not just
To destroy our noble chief they had no right,
It has quenched their spirit greatly,
Diarmaid of the laughing teeth under violent sorrow;
There is not on account of his death banquet or feast;
There will not be peace, there will not be armistice.

Source
"Annals of the Four Masters; http://www.ucc.ie/celt/published/T100005B/index.html
"Irish Kings and High Kings", Francis John Bryne, Dublin 1973.  (from Wikipedia.org)