Why did only Northern Ireland remain part of the United Kingdom after the Irish Free State was recognized?

Northern Ireland remained part of the United Kingdom primarily due to the strong political and cultural desire of its Protestant-majority population to maintain the union with Britain.12

During the early 20th century, as the movement for Irish self-governance grew, unionists in the province of Ulster organized significant resistance to being incorporated into a self-governing Ireland, fearing the loss of their culture and political status under a Catholic-majority government. This resistance led the British government to enact the Government of Ireland Act of 1920, which partitioned the island into two self-governing regions.123

When the Anglo-Irish Treaty of 1921 established the Irish Free State as a dominion within the British Empire, it included a specific clause allowing Northern Ireland to opt out of the new state. Exercising this right, Northern Ireland chose to remain part of the United Kingdom with its own devolved parliament. This decision solidified the partition of the island, a move that was later confirmed by the Boundary Commission agreement of 1925.142