Simplify the following text for a Grade 13 native English-speaking person.
Original: The Union Flag , having remained unaltered following the partition of Ireland in 1921 and the creation of the Irish Free State and Northern Ireland , continues to be used as the flag of the United Kingdom of Great Britain and Northern Ireland .
Simplified: The 1801 design , having remained unchanged despite the partition of Ireland in 1921 and creation of the Irish Free State , continues to be used as the flag of the United Kingdom of Great Britain and Northern Ireland .