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The coat of arms of Munster consists of three gold crowns on a blue field. Similar crowns were included on the arms of Ireland before being superseded by the golden harp in the 16th century. The meaning of the crowns on the flag is not certain, but one possibility is that they may represent three of the medieval Hiberno-Norman lordships in Munster; the O’Briens (Thomond), the Butlers (Ormond) and the Fitzgeralds (Desmond).Similarly, the arms of Munster (dark blue, three antique crowns Or) is thought to have been derived from those of the former Lordship of Ireland, or from the short-lived dukedom of Ireland created for Robert de Vere in 1386. The crowns are now usually depicted as "antique" or "eastern": a gold rim with eight sharp, triangular rays, of which five are seen.

Source: Wikipedia