The United Kingdom of Great Britain and Ireland was primarily a union of parliaments. Administratively there was much less uniformity.

From The Long Shadow: The Legacies of the Great War in the Twentieth Century by David Reynolds.  Page 16.

Where does the United Kingdom fit into these patterns? Essentially as a civic nation (Britain) with an ethnic Achilles’ heel (Ireland). Here the dynamics of nationalism were very different from central and eastern Europe where the explosion occurred in 1918. In the UK, ethnic nationalism was heating up before the Great War but then it simmered down—except in Ireland, where it came to the boil from 1916 with tragic and enduring consequences.

The UK originated as a “composite monarchy” on the pattern of Spain after the dynastic marriage of Ferdinand of Aragon and Isabella of Castile in 1469. Thanks to King Edward I, the English Crown was in control of most of Wales by the late thirteenth century; in 1542, after the English conquest of Ireland, the Irish parliament declared that Henry VIII and his heirs would henceforth be kings of Ireland as well; and in 1603 the childless Queen Elizabeth of England was succeeded by James VI of Scotland, who became James I, King of “Great Britain” (his preferred title was “Emperor”). In time, dynastic fusion was reinforced by political union. The Welsh had already been incorporated (unilaterally) in the English parliament in 1536; likewise the Scots “after negotiating the Act of Union of 1707, and the Irish were included in panic after a major nationalist rising of 1798 that had been backed by an invasion force from Revolutionary France.

The United Kingdom of Great Britain and Ireland, which came into being on New Year’s Day 1801, was primarily a union of parliaments. Administratively there was much less uniformity. Wales had been brought completely under English law and administration by Henry VIII, but the Scots, even after 1707, preserved separate legal and educational systems and their own established Presbyterian Church. After 1801, Ireland also retained its own administrative structure but this was more analogous to the “pro-consular” regimes of the British colonies, with a viceroy, his court, and a tangle of government departments that coexisted uneasily with the Anglo-Irish landed elite imposed on Catholic Ireland as part of the Protestant Ascendancy.