"The IRA of the 60s and 70s was a product of the despicable treaty that the British imposed at the point of a gun. If the British hadn't imposed partition, there would be no Provisional IRA. The entire responsibility lies with the British state. The entire responsibility." - Ken Loach

Ireland was partitioned in 1921 under the terms of the Government of Ireland Act 1920. Six of the nine Ulster counties in the north-east formed Northern Ireland and the remaining three counties joined those of Leinster, Munster and Connacht to form Southern Ireland. Whilst the former came into being, the latter had only a momentary existence to ratify (in United Kingdom law) the Anglo-Irish Treaty that ended the Anglo-Irish War.

Under the Anglo-Irish Treaty, Northern Ireland was provisionally scheduled to be included in the Irish Free State, though it could opt out should the Parliament of Northern Ireland elect so to do. As expected, it did so immediately. Once that happened, as provided for, an Irish Boundary Commission came into being, to decide on the territorial boundaries between the Irish Free State and Northern Ireland. Though leaders in Dublin expected a substantial reduction in the territory of Northern Ireland, with nationalist areas like south Armagh, Tyrone, southern Londonderry and urban territories like Derry and Newry moving to the Free State, it appears that the Boundary Commission decided against this. The British and Irish governments agreed to leave the boundaries as they were defined in the 1920 Act. The Council of Ireland provided for in the Treaty, to link Northern Ireland and the Irish Free State, did not come into being.

The above from Wikipedia

It would appear that nothing was "imposed at the point of a gun" by anybody. If you take a look at British politics you will find that the Liberal Party had been trying to shake Ireland lose and give it Home Rule from the 1870's, contested within Parliament by the Conservatives and by the Lords, and resisted in Ulster by the Protestants, they (the Liberals) almost suceeded in 1914, unfortunately the First World War got in the way.

Maybe Mr. Loach did not research about the "provisional" scheduling of the North joining the South, or of the opt out clause granted to the Parliament of Northern Ireland and agreed to by both parties (British and Irish Governments). The Council of Ireland did not come into being because DeValera wanted no contact with Britain, It has been such a defacto Council of Ireland that has been guiding the Peace Process since the signing of the GFA in 1998.