Act of Supremacy (Ireland) 1537 in the context of Supreme head of the Church of England

The Oath of Supremacy required any person taking public or church office in the Kingdom of England, or in its subordinate Kingdom of Ireland, to swear allegiance to the monarch as Supreme Governor of the Church. Failure to do so was to be treated as treasonable. The Oath of Supremacy was originally imposed by King Henry VIII of England through the Act of Supremacy 1534, but repealed by his elder daughter, Queen Mary I of England, and reinstated under Henry's other daughter and Mary's half-sister, Queen Elizabeth I of England, under the Act of Supremacy 1558. The Oath was later extended to include Members of Parliament (MPs) and people studying at universities. In 1537, the Irish Supremacy Act was passed by the Parliament of Ireland, establishing Henry VIII as the supreme head of the Church of Ireland. As in England, a commensurate Oath of Supremacy was required for admission to offices.

In 1801, retained by the United Kingdom of Great Britain and Ireland, the oath continued to bar Catholics from Parliament until substantially amended by the Roman Catholic Relief Act 1829. The requirement to take the oath for Oxford University students was not removed until the Oxford University Act 1854.

The title of Supreme Head of the Church of England was created in 1531 for King Henry VIII when he first began to separate the Church of England from the authority of the Holy See and allegiance to the papacy, then represented by Pope Clement VII. The Act of Supremacy of 1534 confirmed the King's status as having supremacy over the church and required the nobility to swear an oath recognising Henry's supremacy. By 1536, Henry had broken with Rome, seized assets of the Catholic Church in England and Wales and declared the Church of England as the established church with himself as its head. Pope Paul III excommunicated Henry in 1538 over his divorce from Catherine of Aragon.

Similarly, in Ireland, the Parliament of Ireland passed the Act of Supremacy (Ireland) 1537, establishing Henry as Supreme Head of the Church of Ireland. Again Henry seized assets of the Catholic Church in Ireland and declared the Church of Ireland as the established church with himself as its head. He made himself King of Ireland in 1541, which reinforced his status (previous English kings had held the title of Lord of Ireland, and the island was considered a papal fief based on an 1155 papal bull).