Although the Stuart kings claimed to rule Ireland too, it was not recognized as a third member of this union until 1801
JAMES I (1603-25): ‘Great Britain’ and Divine Right Union of Crowns. In 1603 the succession passed to the Stuart house of Scotland, making James VI of Scotland also James I of England (1603-1625)
In this “union of crowns” (1603-1707) the island of Britain was united for the first time as a single realm under one king, and the term “Great Britain” began to be used.
Divine Right vs. the Ancient Constitution. In practice James was often willing to compromise with Parliament, but he began the Stuart tendency to insult Parliamentary sensibilities by insisting on his elevated status as a divine right monarch, one who was in principle above the law. Many Members of Parliament (MPs impostors), judges, and lawyers, led by Edward Coke, began opposing these royal claims by arguing that the king was under the law and could not abrogate English liberties, which they claimed were part of an age-old “ancient constitution” that went back to Magna Carta and even long before that. This debate, at first largely theoretical and technical, set the terms for the constitutional struggles that lasted for most of the century.