Why did Ireland leave the British Commonwealth in 1949?

Ireland left the British Commonwealth in 1949 primarily to finalize its transition to a fully sovereign republic and to dissolve the remaining constitutional ties to the British monarchy.12

In 1948, an interparty government led by John A. Costello of the Fine Gael party came to power. Costello introduced the Republic of Ireland Act, which repealed the External Relations Act of 1936. This legislative move effectively ended what was viewed as the fiction of Commonwealth membership and established that future relations between Dublin and London would be conducted on the basis of absolute equality between sovereign governments.2

The act took effect in April 1949, formally declaring Ireland a republic and concluding the nation's long-standing effort to remove itself from British domination.12