Northern Ireland: When was it founded?

A policy proposal in the DUP manifesto has started a little of debate over just when Northern Ireland came into existence.

In today’s Irish News, John  Manley talks with Eamon Phoenix and whilst he dismisses the DUP date of 5th May as inaccurate, he still does not have a concrete date for the foundation of Northern Ireland.

Speaking to Manley he said;

There is no reason to mark May 3 – or May 5 for that matter,

He continued going through the various dates that could be used;

:: 20 December 1920 – enactment of the Government of Ireland Act which partitioned Ireland;

:: 22 June 1921 – George V convenes new Northern Ireland parliament in Belfast;

:: 6 December 1921 – Treaty gave north option to opt out of Irish Free State;

:: 7 December 1922 – Northern Ireland Parliament opted out of Irish Free State;

:: 3 December 1925 – Tripartite agreement between Belfast, Dublin and London whereby south recognises border.

However, Phoenix ended taking aim at the DUP date;

No matter what way you look at it that date in the DUP manifesto is not only wrong in terms of the century and millennium but it’s also wrong in terms of historical events

My question to the Sluggerverse is this, can any of you shed some light on this or is there no exact date?