Officially, the actual division of the island of Ireland between Northern Ireland and the Republic of Ireland happened in the Government of Ireland Act 1920, which officially went into effect in 1921. On the other hand, that partition was part of an act which reflected a political situation from before World War I which had passed by 1921. The act divided the island into two "home rule" areas of "Northern Ireland," which wanted to be a constituent element of Great Britain, and "Southern Ireland," which was well on its way to independence as the Irish Free State. Nonetheless, the divisions that would dominate Irish politics and events for the rest of the century were set in stone by the Partition of Ireland.
Which is What Happened Today, May 3, in 1921.