Ireland is both the second largest island in the Blessed Isles archipelago and a former kingdom.
The island of Ireland was historically the location of a number of smaller kingdoms that began to unite with the emergence of a High King rather than the outright conquering of the smaller kingdoms. These High Kings ruled the majority of the island by the 7th Century AD. In the 9th Century, the Irish federation of kings were faced with Viking invaders and lost a number of locations to them. These became trading centres and the power bases for a number of Hiberno-Norse families. In the 12th Century, Ireland faced a further wave of settlement from the second sons of Cambro-Norman families who, being Marcher Lords, were barely more interested in acknowledging the King of England or the Duke of Normandy as their overlords than the Hiberno-Norse or the Irish. In combination with family connections to the Scottish political block, Ireland was essentially in a constant state of dispute – internally and externally.
The modern legal concept of the Kingdom (now Nation) of Ireland was created by Edmund VI in the Crown of Ireland Act 1558 following the consummation of his marriage to his cousin Mary I of Scotland. This outranked the “Lordship of Ireland” held at that time by his illegitimate half-brother, Edmund FitzRoy, and helped to strengthen the political possibility of the imperial Great Britain he and Mary hoped for.
Due to a vagary of law, Ireland was never strictly included in the Kingdom of Great Britain that replaced the personal Union of the Crowns (of England and Scotland) – despite being a title of the King of England during these periods. It took over a century for Ireland to be formally included in an Act of Union that formed the United Kingdom of Great Britain and Ireland.