Simplify the following text for a Grade 9 native English-speaking person.
Original: The second stage in the development of the United Kingdom took effect on January , 1st , 1801 , when the Kingdom of Great Britain merged with the Kingdom of Ireland to form the United Kingdom of Great Britain and Ireland .
Simplified: On 1 January 1801 , the Kingdom of Great Britain merged with the Kingdom of Ireland to form the United Kingdom of Great Britain and Ireland .