The UK government promised the Barnett Formula would remain but billions have been pledged to other parts of the UK, bypassing the formula, as part of political pacts with the DUP.
In the case of the £1 billion for Northern Ireland the Barnett Formula didn’t apply because public spending in England didn’t change.
"The Westminster parties... promised the Barnett Formula would remain but billions have been pledged to other parts of the UK—bypassing Barnett—as part of political pacts with the DUP."
The Barnett Formula is a system which decides, following public spending being allocated to England, how much additional money should go to devolved governments in Scotland, Wales and Northern Ireland, based on population.
It isn’t strictly right to say the formula was “bypassed”. According to the government’s own guidance, it couldn’t have been applied in the case of the £1 billion grant to Northern Ireland that the DUP negotiated from the government in 2017, because it didn’t involve a change to spending in England.
The government is not legally required to apply the formula, it is an established convention that it does so.