from 1801 to 1921, all of ireland was part of the same country, called the united kingdom of great britain and ireland. in 1919, a war broke out, the irish war of independence, and on december 6 1921, the irish free state became independent. after a new constitution came into effect in 1937, the state became a republic. northern ireland stayed with the uk, and this would lead to the troubles beginning in the 1960s and ending with the good friday agreement signed in 1998.