When did northern ireland and southern ireland separate?
No context provided
When was the act passed, which separated Northern and Southern Ireland?
December, 1920
No context provided
When did the act go into effect, which separated Northern and Southern Ireland?
May 3, 1921
Partition of Ireland
https://en.wikipedia.org/wiki/Partition%20of%20Ireland
The partition of Ireland (Irish: críochdheighilt na hÉireann) was the process by which the Government of the United Kingdom of Great Britain and Ireland divided Ireland into two self-governing polities: Northern Ireland and Southern Ireland. It was enacted on 3 May 1921 under the Government of Ireland Act 1920. 
Partition of Ireland
The partition of Ireland was the process by which the Government of the United Kingdom of Great Britain and Ireland divided Ireland into two self-governing polities, Northern Ireland and Southern Ireland.  The act was called the Government of Ireland Act 1920, and it was passed in December, 1920. On May 3, 1921, it was enacted. 