Ireland has been in personal union with…

Ireland has been in personal union with England since 1541, when the Irish Parliament had passed the Crown of Ireland Act 1542, proclaiming King Henry VIII of England to be King of Ireland. (Before then, since the twelfth century, the King of England had been overlord of the Lordship of Ireland, a papal possession.)

Both Ireland and England had come in personal union with Scotland with the Union of the Crowns in 1603.

In 1707, the Kingdom of England and the Kingdom of Scotland had been united into a single kingdom: the Kingdom of Great Britain.

The Irish parliament at that time was subject to a number of restrictions that placed it subservient to the Parliament of England (and following the union of England and Scotland, the Parliament of Great Britain).

In the century that followed, the union of England and Scotland, Ireland gained effective legislative independence from Great Britain through the Constitution of 1782.

However, the rebellion in 1798, involving a French invasion of Ireland and the seeking of complete independence from Great Britain, has descended into sectarian violence and ruthless repression by the Irish authorities.

The Act of Union 1800, to come into effect on January 1, 1801, will link Britain and Ireland legislatively.

The Acts of Union, were passed on July 2, 1800, and August 1, 1800, respectively, are two complementary Acts, namely:
    The Union with Ireland Act 1800 (39 & 40 Geo. 3 c. 67), an Act of the Parliament of Great Britain, and
    The Act of Union (Ireland) 1800 (40 Geo. 3 c. 38), an Act of the Parliament of Ireland.

The British drive for union between Great Britain and Ireland that passes in 1800 is motivated at least in part by a desire to restore order to Ireland, end sectarianism (including the introduction of Catholic Emancipation) and to ensure that Ireland will not become a staging post for a French invasion of Great Britain.