Whiter the Boundary Commission in this scenario?
It will be there as in OTL, and with probably the same results: the North does not take part (they have nothing to gain); some in the the South will expect large changes. Eventually small ones are proposed under protest, and none implemented.
Of course the situation is unlikely for the reasons I gave before, but if Westminster insists on the whole Province in the Treaty then the only territory that might change hands later would be in Cavan, which is quite isolated from the rest of Ulster. Once they have it, the North are not going to to want to give up the Monaghan rail link, or any part of Donegal.
There are several interesting consequences:
The most northerly part of Ireland is now actually in the North
The great majority of the land, if not the population, is now west of the Bann. I can see Derry developing as a counterweight to Belfast, together (possibly) with Omagh, Enniskillen and Monaghan. Politics may become more divided East/West than on sectarian lines.
The west includes the pilgrimage site of Lough Derg and the Gaeltacht of West Donegal, so Catholic Gaelic culture cannot be associated solely with the Free State. Interest in the language was not so politicised as it became later in OTL - Lord Carson, for example was a fluent Irish speaker. Perhaps the northwest becomes a thriving centre for Gaelic culture, with or without the blessing of Stormont.
Perhaps, if things progress more or less as in OTL, there will be a Royal Navy base at Killybegs to make up for the loss of the Treaty ports?
All in all TTL Donegal could be a very interesting place