The Flag of Leinster consists of an Irish harp in gold on a green background: vert a harp or stringed argent. The arms of Leinster are (among other examples) represented on the flag of the Provinces of Ireland, as well as in a stylised version in the logo of the Leinster Rugby team. The flag is often used as a symbol of Irish nationalism and representitive of the island as a whole.
Possibly the oldest instance of the use of the harp device on a green field was the flag of Owen Roe O'Neill. Owen Roe, nephew of Hugh, had entered the Spanish service after his uncle's defeat at Kinsale in 1601. Owen rose to prominence in the Spanish army, and in 1642 returned to Ireland to assist the Irish Confederation in the war that broke out the previous year. It is recorded that his ship, the St Francis, as she lay at anchor at Dunkirk, flew from her mast top "the Irish harp in a green field, in a flag". Because the confederation's headquarters were located in Kilkenny – the principal city of Leinster "without the pale" – his flag may have had a special significance for that province. The Confederation seal also incorporated, among a number of other motifs, a representation of the Irish harp.