The Union of 1707 which created the Kingdom of Great Britain was not the final chapter in uniting the Britons.
The building of the United Kingdom was completed almost 100 years later with the Union of Great Britain and Ireland. The circumstances were rather different and the world and the times more troubled, but the Union and the equality of Ireland with the other British lands was long overdue. The Parliaments of Great Britain and of Ireland each passed an Act of Union in 1800, taking effect "upon the first Day of January which shall be in the Year of our Lord one thousand eight hundred and one".
Thus at the opening of the new year and of the new century, on 1st January 1801, the United Kingdom of Great Britain and Ireland was born.
The Union was enacted by twin Acts of the two Parliaments of Ireland and then of Great Britain, of which the Irish Parliament's Act was later given the short titles "The Act of Union (Ireland) 1800", and that of Great Britain "The Union with Ireland Act 1800". The complete original text of the Acts is set out here.
The Act of Union (Ireland) 1800 - full original text
The Union with Ireland Act 1800 - full original text
The two Acts have born some amendment in the last two hundred years, as temporary provisions and superseded references have been formally repealed. The heart-oak remains though.
(The Act of Union (Ireland) 1800 in current form as amended on the Statute Law Database)
(The Union with Ireland Act 1800 in current form as amended on the Statute Law Database)
The Union of 1801 has not been undone, notwithstanding that in 1921 parts of Ireland left the kingdom to form the Irish Free State and latterly the Irish Republic, so that the United Kingdom is now known as "the United Kingdom of Great Britain and Northern Ireland". The Union of 1800 was the foundation of more than a country. It created a new kingdom; the United Kingdom, and the United Kingdom created the modern world.