The Boundary Commission was established by the Anglo-Irish Treaty that ended the Anglo-Irish War in 1921. Its purpose was to decide on the precise delineation of the border between the Irish Free State, which had seceded from the United Kingdom of Great Britain and Ireland, and Northern Ireland which was to remain part of the new United Kingdom of Great Britain and Northern Ireland. While nationalists hoped for a considerable transfer of land from Northern Ireland to the Free State (reflecting the wishes of people who lived along the new border), the Northern Ireland government obstructed the establishment of the Commission, resulting in the British government assigning a representative to represent their interest.
When the Commission decided on a very small net transfer of land to Northern Ireland (the reverse of what was expected), its conclusions were leaked to the Morning Post in 1925, causing protests from both the unionists and nationalists. In order to avoid the possibility of further disputes, the British, Irish, and Northern Ireland governments agreed to suppress the report, and the existing (Government of Ireland Act 1920) border was ratified by W.T. Cosgrave, Sir James Craig, and Stanley Baldwin in December 1925.

The provisional border 1920 – 1925
After a clause providing for Northern Ireland (as defined by the Government of Ireland Act 1920 to opt out of the new Free State, the remainder of Article 12 declares,[6]
Provided that if such an address [exercising the opt out, as expected] is so presented, a Commission consisting of three persons, one to be appointed by the Government of the Irish Free State, one to be appointed by the Government of Northern Ireland, and one who shall be Chairman to be appointed by the British Government shall determine in accordance with the wishes of the inhabitants, so far as may be compatible with economic and geographic conditions the boundaries between Northern Ireland and the rest of Ireland, and for the purposes of the Government of Ireland Act, 1920, and of this instrument, the boundary of Northern Ireland shall be such as may be determined by such Commission.
Accordingly the Free State established the North-Eastern Boundary Bureau which had prepared 56 boxes of files to argue its case by 1925.

Article 12 of the Treaty
Due to the delay caused by the Irish Civil War, it was not until 1924 that the Commission was appointed. The Northern Ireland government, which adopted a policy of refusing to cooperate with the Commission since it did not wish to lose any territory, refused to appoint a representative. Ultimately the Labour government in Britain legislated to allow itself to impose a representative on their behalf in order to enable the procedure to go ahead. The Commission was convened in 1925 consisting of:

Justice Richard Feetham of South Africa as Chairman (appointed by, and representing, the British Government)
Eoin MacNeill, Minister for Education (appointed by, and representing, the Free State Government)
J.R. Fisher, a Unionist newspaper editor (appointed by the British government to represent the Northern Ireland government) Negotiation
In the background, under the terms of the 1921 Anglo-Irish Treaty the Irish Free State had agreed to assume responsibility for a part of the Imperial debt. This had not been paid by 1925, in part due to the heavy costs incurred in and after the Irish Civil War of 1922-23. A tacit agreement was reached that the 1920 boundary would stay as it was, and in return, Britain would not demand payment of the amount agreed under the Treaty. Since 1925 this payment was never made, nor demanded.

Imperial debt
In the Dáil debates on the outcome on 7 December 1925, Cosgrave mentioned that the sum due under the Imperial debt was not yet fixed but was estimated at £5m. to £19m. annually, Britain having a debt of £7 billions. Cosgrave's aim was to eliminate this amount: I had only one figure in my mind and that was a huge nought. That was the figure I strove to get, and I got it.

Notes

Partition of Ireland
History of Ireland
History of Northern Ireland
History of the Republic of Ireland
Anglo-Irish Treaty
Republic of Ireland-United Kingdom border
Repartition of Ireland
Irish Government Bill 1886 (First Irish Home Rule Bill)
Irish Government Bill 1893 (Second Irish Home Rule Bill)
Government of Ireland Act 1914 (Third Irish Home Rule Act)
Government of Ireland Act 1920 (Fourth Irish Home Rule Act)