The Twelve Days war was a military conflict between the Duchy of Leinster and the County of Tir Eoghain.
After the War of the Harp, the County of Tir Eoghain had remained independent from Leinster and claimed to be the successor to the Duchy of Ulster. Murchaid Ui Mordha, duke of Leinster and later King of Ireland, saw this as both an annoyance and a threat upon his fathers honor.
Murchaid had the armies of Ulster and Dublin marched within twelve miles of the capital of Tir Eoghain, while feeding disinformation to the Count of Tir Eoghain's spymaster. As soon as the troops reached this position, Leinster sent a declaration of war.
Although the troops had come prepared for a siege, a traitor hired by the Spy Master of Leinster opened the south gate. The forces of Leinster immediately swarmed into the castle and slew the Count.
After the loss of its leader, Tir Eoghain surrendered and became a county of the Duchy of Leinster. It would later be transferred to Ulster after the formation of the Kingdom of Ireland.