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The flag of the Irish province of Leinster is a banner with the provincial coat of arms: a gold Irish harp with silver strings on a green field (blazon: vert a harp or stringed argent). These arms are similar to the arms of Ireland, which have the same device on a field of blue rather than green. The arms (Vert, a Harp Or, stringed Argent) is believed to have likely evolved from the arms of Ireland itself with a change of tincture.

Source: Wikipedia