In the first millennium, the island of Ireland was a complex patchwork of rival kingdoms. Among the most powerful were Munster in the southwest, Leinster in the east, and the lands of the Ui Neill in the north. Vikings added their own influence to the lot with settlements at Limerick, Cork, and Dublin, their largest. Even though the Irish kingdoms constantly attempted to raid and outmaneuver one another with alliances and counter-alliances, there was a titular High King that held the highest authority in Ireland. In AD 980, Mael Sechnaill, King of Meath (the most southerly Ui Neill) maintained a delicate balance as High King, securing his place by halting the growth of Viking power in the Battle of Tara.
In 1002, the balance shifted when Mael Sechnaill's kinfolk in the north began to support the growing strength of Brian Boru, king of Munster. Mael Sechnaill was amicable with Brian Boru, seeing him as an ally and rival more than an enemy since they had both worked to defeat uprisings in Leinster under Mael Morda and the Viking ruler of the Kingdom of Dublin, Sigtrygg Silkbeard. Morda's mother Glormfaith, former wife of Sechnaill, was mother of Sigtrygg and later became the wife of Brian Boru until their divorce in the early second millennium.
Although Brian Boru had worked for a decade to secure his rule, an uprising broke out under Flaithbertach, king of the more northern Ui Neill who had been first in the line of succession to Sechnaill. Brian Boru swiftly put down the rebels, but it gave a chance for Morda and Sigtrygg to rebel themselves; the annals say Glormfaith’s sense of vengeance drove much of the political energy behind the rebellion. In 1013, Brian Boru and his allies invaded Leinster while the rebels’ unsuccessful raids took place on Meath and along the southern coast. That winter, Sigtrygg called up international support from Vikings at Orkney and Man, collecting a massive force at Dublin.
When spring came and campaign season began, the two armies met at Clontarf, west of Dublin, at dawn on Good Friday. Brian Boru prayed while his armies ravaged the landscape and put it to the torch. The action drew out the Viking-Leinster army, the hardened foreign mercenaries on the front lines followed by the Dubliners and the Irish. Brian's son Murchad and his fifteen-year-old grandson Toirdelbach led his Irish armies while the aged king continued to pray in his tent. His prayers were interrupted when Brodir, a Viking warrior from the Isle of Mann, attacked.
Yet Brian Boru proved quick, especially for a septuagenarian, and ran Brodir through. He determined to march out to the battlefield, which was soaked with the blood of slain thousands. The Vikings had superior mail armor, but it proved useless when the Irish drove them into the sea that evening. Brian Boru’s late charge relieved his son Murchad, who legendarily killed fifty men with each of his duel-wielded swords. As the battle ended, Brian Boru kept his grandson from pursuing the Vikings foolhardily into the high tide, where hundreds drowned.
With his authority secured, Brian Boru determined that he could continue to rule only through his son and grandson. He spent his final years tutoring them and establishing centralized training grounds for national armies. As Murchad and Toirdelbach came to the Irish kingship, they found fast allies against the common enemy Vikings with the Normans, who recently conquered England. Soon after 1100, Irish princess Blathmin married Stephen to become the queen consort of England, the first of many Irish rulers over the neighboring English.
The unified Irish were among the first to follow Norman knights to the Crusades. Over the next decades, the campaigns would ultimately fail despite the combined forces of Christendom, but the returning crusaders brought spices and technology that caused a hunger for more trade. Medieval Ireland had built a sophisticated system of investment and shipwrights to transport their warriors to the Middle East, and these were turned to an Irish merchant navy.
After being driven out of the Mediterranean by Italian fleets, the Irish turned south toward the African coast, battling the growing Portuguese Empire. The few Irish trading posts there were given up after the discovery of a new world across the Atlantic, where the Irish kingdom became wealthy through colonies in more temperate areas, capitalizing on the fur trade. After centuries of wealth, those colonies, too, would declare their independence, and Ireland would settle into its famously neutral role with an economy kept separate from the European Zone.
In reality, Brian Boru was killed at Clontarf, as were his heir Murchad and Murchad's son, Toirdelbach. While the battle secured Irish authority over the island and severely weakened Viking power, an entire would-be dynasty over Ireland was wiped out. Mael Schnaill returned to the High Kingship, which would soon fall again to squabbles and constant upheaval. In 1169, the Normans from England invaded, breaking Irish kingship. It would be the first of many English waves of conquest over the Ireland, which would not win its independence until 1922.