hi! not discouraging a direct response, but you may find what you need in these previous posts

• How was the border of Northern Ireland decided on?

• Could anyone give me a brief, modern history of the conflicts between Ireland, Northern Ireland, and the UK? - warning: it's not brief :)

• Why did the UK make more of an effort to hold onto Northern Ireland, when arguably, made less efforts to keep other land holdings?

Look at the links provided by /u/Searocksandtrees for a more detailed answer. However, here is the very brief rundown.

The "independence movement" did not collectively agree. The terms of the 1921 Anglo-Irish Treaty to end the Anglo-Irish War called for the division. The history of this treaty itself is incredibly complex, full of specific issues that divide many historians. Essentially, what became Northern Ireland with the 1920 Government of Ireland Act (passed by the UK Parliament) was given the option to either join or remain separate from the newly created Irish Free State.

Response to the Treaty was...divided. In fact, the Irish Civil War of 1922-1922 was fought over the Treaty, with one of the major points of contention being the division of the island.

The other links will give you more depth as I am on my phone and can't provide you the detail you may desire.