I have seen, not the movie, but the comment.

Rating: Null/10 (17 June 2006)

The Irish Civil War of 1922-23 arose not from the partition of Ireland but from an oath of loyalty to the British Monarch as head of the Commonwealth required under the terms of the 1921 Articles of Agreement of members of the Dail. This oath was not required of the IRA. The Dail split, 64 for the treaty, 57 against and the army under its authority the IRA split also.
The anti-treaty people did not want an all-Ireland "Free State". The words "Free State" are the dirtiest words in the Republican lexicon.
The "Treaty" held out the prospect of unity, and both pro-Treatyies and anti regarded Partition as a very temporary expedient by the British in 1920, too ludicrous to have any future.