The Parliament of Great Britain, on 2 July 1800, passed An Act for the Union of Great Britain and Ireland, also known as The Union with Ireland Act 1800, which came into effect on 1 January 1801. The Parliament of Ireland also, on 1 August 1800, passed a complementary Act of Union (Ireland) 1800; both in effect created a Kingdom of Great Britain and Ireland. The Acts also created a united parliament in London.
The Kingdom of Great Britain had been created in 1707 when the Kingdom of England and the Kingdom of Scotland were united by the Union with Scotland Act 1706 and the Scottish parliament’s Union with England Act 1707, both of which came into effect on 1 May 1707.
The Royal Proclamation on 1 January 1801 referred to the arms and flag of the United Kingdom, stating:
… that the Union Flag shall be Azure, the Crosses Saltires of St. Andrew and St. Patrick Quarterly per Saltire, counterchanged Argent and Gules; the latter fimbriated of the Second, surmounted by the Cross of St. George of the Third, fimbriated as the Saltire …
At 0930 hours on the morning of 1 January 1801, a militia regiment, The Royal Tyrone Regiment, marched their new Colours, displaying the latest Union Flag, down to the Dublin Castle Guard mounting escorted by the Grenadier Company, with the officers in full ceremonial dress complete with bearskin headdress. This was the first regiment to mount guard at Dublin Castle in the new era of the United Kingdom of Great Britain and Ireland.