With wealth from Dublin, and an ally in his nephew
Gilla Pátraic, the king of Osraige, Diarmait attacked
Donnchad in 1054, raiding Emly and Duntrileague. He
sent a naval expedition to raid Scattery Island in 1057.
His campaigns received reinforcement in 1058, when
Diarmait made an alliance with his foster son, and
Donnchad’s nephew, Tairrdelbach Ua Briain. The allies
raided Limerick, and Diarmait defeated Donnchad in
a battle fought at the Galtee Mountains. The next year
Diarmait returned to Munster in order to destroy
Donnchad’s strongholds. Now it was Donnchad’s turn
to submit to Diarmait. A decisive battle was fought in
1062 at Cleghile (Co. Tipperary), where Diarmait and
Tairrdelbach defeated Donnchad’s army. At the height
of his power, Diarmait used his new position as king-
maker to indulge in political assassination in 1066,
when he and Tairrdelbach paid the Connacht prince
Áed Ua Conchobar thirty ounces of gold to kill a rival.
They invaded the province in the next year.
Since 1052, Diarmait had become increasingly
involved in adventures outside Ireland, either directly
or through assistance to foreign lords. The English
nobleman Ælfgar, the earl of Mercia, fled to Ireland,
almost certainly to Diarmait, in 1055 and was supplied
with a fleet. In 1058, Diarmait allied with the Norwe-
gians on an expedition into western England. This was
led by the Norwegian heir-apparent Magnus, son of
the Norwegian king Harald “Hard-Counsel.” Only
troops from Dublin were present for that enterprise,
but they were joined at the last minute by Ælfgar and
the Welsh prince Gruffudd ap Llywelyn. That venture
might have suggested other endeavors to Diarmait. In
1061, Murchad led a fleet to the Isle of Man, where
he defeated his father’s old foe Echmarcach and col-
lected taxes from the kingdom of the Isles. The Welsh
prince Gruffudd ap Llywelyn might have fled to
Diarmait’s court after he had been forced from Wales
in 1064 by Harold Godwinson; within a generation
there were circulating stories that he had been slain in
Ireland in circumstances of treachery. After his friend
Harold Godwinson was defeated and slain in October
1066 at the battle of Hastings, Diarmait’s court became
one of the centers for the Anglo-Saxon resistance. He
gave refuge to Harold’s sons and supplied fleets for
their raids on England in 1068 and 1069.
Diarmait’s final years saw the crumbling of his
empire. His sons Murchad and Glúniairn died in 1070;
Murchad died on March 21 of a plague that ravaged
Dublin. Elderly and without the assistance of his sons,
Diarmait could not maintain order even in Leinster.
Tairrdelbach Ua Briain was forced to come to his foster-
father’s aid in 1070 and 1071. One center of unrest was
Diarmait’s own family. His grandson Domnall (the
son of Murchad) and his nephew Donnchad (the son
ofDomnall Remar) competed for supremacy, and
Tairrdelbach was forced to intervene in order to pre-
vent open warfare. In 1071, Diarmait made his final
visit to Tairrdelbach, leaving his blessing on Munster.
Leinster affairs were quiet enough in 1072 that Diarmait
felt able to lead an expedition into Meath. This was his
last miscalculation, for in the battle of Odba, fought on
February 7, he was defeated and executed by Conchobar
Ua Máelshechnaill. In his obituary the so-called Annals
of Tigernachdescribe Diarmait as king of the Welsh,
the Isles, Dublin, and the southern half of Ireland.
Diarmait’s career shows that early Irish kings were
not deliberately insular and provincial. He was an ardent
student of his opponents’ tactics and methods, using
Dublin’s fleet as astutely as any Viking prince. Diarmait
clearly understood how vital were the Viking trading
centers for political supremacy in Ireland. His rise to
power directly matched the acquisition of economic
resources in Dublin. These Viking towns were also inter-
national ports, and they provided the avenue for Diarmait
into the wider world of Irish Sea power politics through
alliances with English, Norwegian, and Welsh princes.
This is not to ignore Diarmait’s importance for the
revival of his family’s fortunes in Leinster and Ireland.
After Diarmait, Uí Chennselaig was the dominating
force in the province and one of the most powerful
families of eastern Ireland. Within Ireland, Diarmait
assembled an impressive network of alliances that
went from the Ulaid in the north to Osraige in the
south, across to Dál Cais and Connacht in the west.
Ironically, it was not his grandiose empire building
outside Ireland that brought about Diarmait’s end, but
a typical and petty conflict with his neighbors.
BENJAMIN HUDSONReferences and Further Reading
Ó Corráin, Donncha. Ireland before the Normans. Dublin: Gill
& Macmillan, 1972.
Ó Corráin, Donnchad. “The Career of Diarmait mac Máel na
mBó, King of Leinster,” Journal of the Old Wexford Society
3 (1970–71): 27–35, and 4 (1972–73): 7–24.
Ryan, John. “Pre-Norman Dublin.” Journal of the Royal Society
of Antiquaries in Ireland79 (1949): 73–88.
Stokes, Whitley. “The Annals of Tigernach: The Fourth Frag-
ment,A.D. 973–1088.” Revue Celtique17 (1896): 336–420.
See alsoDublin; Fine Gall; Laigin; Leinster;
Leth Cuinn and Leth Moga; Mac Murchada;
Marianus Scottus; Ua Briain, Tairrdelbach;
Uí ChennselaigDICUIL
Dicuil (c.760–post 825) was an Irish scholar-exile at
the courts of Charles the Great and Louis the Pious,
and an important author of several works on geogra-
phy, computus, grammar, and astronomy. The onlyDIARMAIT MAC MÁELE-NA-MBÓ (Reigned 1036–1072)