badge of the kingdom of ireland
a monarchical system of government existed in ireland from ancient times until—for what became the republic of ireland—the early twentieth century. northern ireland, as part of the united kingdom, remains under a monarchical system of government. the gaelic kingdoms of ireland ended with the norman invasion of ireland, when the kingdom became a fief of the holy see under the lordship of the king of england. this lasted until the parliament of ireland conferred the crown of ireland upon king henry viii of england during the english reformation. the monarch of england held the crowns of england and ireland in a personal union. the union of the crowns in 1603 expanded the personal union to include scotland. the personal union between england and scotland became a political union with the enactments of the acts of union 1707, which created the kingdom of great britain. the crowns of great britain and ireland remained in personal union until it was ended by the acts of union 1800, which united ireland and great britain into the united kingdom of great britain and ireland from january 1801 until december 1922.
after that date, most of ireland left the united kingdom to become the largely independent irish free state, a dominion within the british empire; the remaining part, northern ireland, remained within the united kingdom. both the free state and the united kingdom, which changed its name to the united kingdom of great britain and northern ireland in 1927, had the same person as monarch: george v. in 1937, the year after george v's death, the free state adopted a new constitution which changed the state's name to ireland (or Éire) and removed all mention of the monarch. in april 1949, ireland was declared a republic, with the description of the republic of ireland, and it left the commonwealth of nations. since april 1949, the only part of the island of ireland that has retained a monarchical system is northern ireland (as part of the united kingdom of great britain and northern ireland).