The War of the Harp was a military conflict between the Duchy of Leinster and the Duchy of Ulster.
In 1057, the Duke of Leinster gained a legitimate claim on the title Duke of Ulster. As the current duke of Ulster was widely unpopular, the Duchy of Leinster seized the opportunity. The armies of Laigan, Dublin and the Isle of Mann marched north to Ulster.
There was virtually no resitance until the armies of Leinster reached Belfast. After a pitched battle, the far outnumber army of Ulster was shattered.
Belfast was then placed under siege for six months, until it surrendered. Although this was a considerable strain on the rapidly failing economy of Leinster, the territorial gains where enough to stave of complete ruin. The Duchy of Leinster now controlled the majority of the Irish coastline on the Celtic Sea.