Union Jack with St. Patrick's Cross

The flag of England is the Cross of St. George - a red cross on a white background.  The flag of Scotland is the Cross of St Andrew - a white diagonal cross on a blue background.  Those flags were combined to form the first Union Flag - the flag of Great Britain - in 1707.

After the 1800 Act of Union, in which the Kingdom of Ireland and the Kingdom of Great Britain were merged, the Cross of St. Patrick - a red diagonal cross on a white background - was added to the Union Flag.   That is the flag, depicted above, which today remains the flag of the UK (United Kingdom).