Although others later claimed the title, the last native Ard Rí (High King) is often stated to have been Ruaidhrí Ua Conchobair (in English, Rory or Roderick O'Connor), a member of the Ua Conchobair dynasty who were kings of Connacht and Ireland at the time. He died in the twelfth century. Although British control over the country did not become firmly established until the seventeenth century, Norman forces controlled the Pale, an area around Dublin, from the late twelfth century, and it became difficult for any Irish king to gain control over the entire island. Although they might be regarded as usurpers, the kings and queens of Britain began to style themselves as monarchs of Ireland also.
Ireland became the Irish Free State with the settlement between the Irish rebels and the British in 1922. Although Ireland thus became an independent state, it was still part of the British Commonwealth, and the King of England was also nominally King of Ireland (just as the current British monarch is still known as the Queen of Canada and Australia). With the Westminster Statute in the 1930s, and the renaming of the Irish Free State as "Ére," these places became "dominions" -- that is, effectively, separate kingdoms who all shared the same person in the role of king: so, George V was King OF Ireland, as well as King OF Canada, OF Australia, and OF Great Britain. In 1936, when Edward VIII abdicated (to marry Wallis Simpson), his abdication did not become effective until it was accepted by Parliament the next day, but there is a little known fact that Dáil Éireann (the parliament of Ireland) did not get around to accepting the abidication until two days later, so for one day, Edward was actually King of Ireland without being king of anything else. George VI succeeded him, and was technically King of Ireland until 1948 when Éire was officially declared by its government to be a republic, and so the last ties (which by that time were meaningless anyway) with the crown were broken. However, today, Northern Ireland is still part of the United Kingdom, and so Elizabeth is queen "of the United Kingdom of Great Britain and Northern Ireland