Gillomanius was the King of Ireland in the age before Arthur, when the island of Britain was still divided between Britons, Saxons, and Picts, and when Ireland itself was a land of rival kingdoms and ancient sacred sites. His realm included Mount Killaraus, a mountain in Ireland where a stone circle called the Giant's Dance stood, a monument of such antiquity that Merlin the prophet claimed giants had transported the stones from the remotest confines of Africa and erected them for the healing of their sick. The stones were enormous, beyond the strength of any mortal to move, and they possessed medicinal properties that made them sacred to the Irish people. When Aurelius Ambrosius, the last Roman king of Britain, sought to build a memorial to the four hundred and sixty British nobles who had been treacherously slain by Hengist at the Treachery of the Long Knives, he turned to Merlin for advice. Merlin told him that no stones in Britain would suffice. Only the Giant's Dance on Mount Killaraus in Ireland would stand forever as a fitting monument, and only Merlin possessed the knowledge and art to move them.

Aurelius sent his younger brother Uther Pendragon with fifteen thousand men to fetch the stones. When Gillomanius heard that the Britons had arrived in Ireland with the intention of taking the Giant's Dance, he gathered his army and marched against them. The battle was fierce, for the Irish king was determined to defend a monument that his people had held sacred for generations. But Uther was a warrior of exceptional skill, and the Britons were fighting not merely for stones but for the honour of their dead. Gillomanius was defeated. His army broke and fled, and the Britons advanced to Mount Killaraus. There Merlin worked his magic, dismantling the stone circle with an ease that made the labour of fifteen thousand men seem like child's play. The stones were loaded onto ships, transported across the Irish Sea, and raised at Amesbury around the burial mound of the murdered nobles. The monument became Stonehenge, and Gillomanius was left with an empty mountain and a burning grievance.

That grievance would find its expression in the person of Pascentius, the third and last surviving son of Vortigern, the British king who had invited the Saxons into Britain and who had been burned alive in his tower by Aurelius Ambrosius. Pascentius had fled to Germany after his first invasion ended in defeat, but he could not return there in shame. Instead he sailed to Ireland, where Gillomanius received him with sympathy and a shared hatred of the Britons. The Irish king listened to Pascentius's tale of woe, and the two men saw in each other's grievances the makings of an alliance. Pascentius wanted to avenge his father and reclaim the throne of Britain. Gillomanius wanted to punish the Britons for the theft of his stones and the humiliation of his defeat. They fitted out a fleet, gathered their armies, and sailed for Menevia, the city that would one day be called Swansea, on the western coast of Wales.

The timing could not have been more favourable to their cause. Aurelius Ambrosius lay seriously ill at Winchester, struck down by a sickness that had weakened him at the very moment his enemies were gathering. The news that the Irish king and the Saxon prince had landed with a powerful army spread through Britain like wildfire. Uther, the king's younger brother, took it upon himself to defend the realm. He gathered the army of the Britons and marched into Cambria to meet the invaders, taking Merlin with him as adviser. Pascentius and Gillomanius rejoiced when they heard that Aurelius was incapacitated, believing that the sickness of the British king would make their conquest all but certain.

The two armies met on the field before Menevia, and the battle that followed was bloody and fierce. Both sides fought without quarter, the Irish and the Saxons desperate to avenge old wrongs, the Britons determined to defend their land and their dying king. As the day wore on, the advantage shifted to the Britons. Uther fought with the fury of a man who knew that his brother's life and his kingdom's future hung in the balance. Towards the close of the day, the Britons broke through the enemy lines. Gillomanius fell, struck down on the field where he had hoped to win back his honour. Pascentius fell beside him, the last son of Vortigern dying as his father had died, by British hands. With the deaths of their leaders, the Irish and Saxon army broke and scattered. The Britons pursued them to their ships, killing all who were caught, and the invasion was ended.

Uther rested and savoured his victory, but before the sun had set on that day of triumph, a messenger arrived from Winchester with grievous news. Aurelius Ambrosius was dead, poisoned by the false physician Eopa while Uther had been fighting on the western coast. The bishops of the land had buried the king with royal magnificence inside the Giant's Dance, the very monument that Gillomanius had tried to defend. Uther was crowned king in his brother's place, taking the name Pendragon after a dragon-shaped comet that had appeared in the sky before the battle, a sign that Merlin had interpreted as the passing of one king and the rise of another. From Uther Pendragon would come Arthur, the greatest king Britain would ever know. And the road to that kingship had been cleared, in part, by the defeat and death of the Irish king who had tried to keep his stones and lost everything in the attempt.

Gillomanius disappears from the chronicles after Menevia, his name never spoken again in the histories of Britain. He is not a villain of the stature of Hengist, who butchered nobles at a peace feast, nor of Mordred, who betrayed a kingdom. He is something more poignant: a king who defended what was his, who lost it to magic and superior force, and who threw his lot in with a desperate Saxon prince rather than accept the humiliation of defeat. His legacy lies in the stones themselves, in the monument that still stands on Salisbury Plain, and in the modern archaeological discovery that the bluestones of Stonehenge did indeed come from the west, from the Preseli Hills in Wales, a finding that gives a strange grain of truth to the legend that Merlin told. The Irish king who tried to keep his stones may have been defending something more real than Geoffrey of Monmouth knew.