At most you could get seven counties into Northern Ireland: the six plus Donegal (as I recall the Boundary Commission actually intended to give a largish bit of Donegal to Northern Ireland, one reason for its less than approving reception from the Free State and Britain), but six seems the most likely figure. Many in Britain wanted even less than that, Asquith himself proposed excluding just four Counties from Home Rule (Antrim, Armagh, Down and Derry, and South Armagh would likely have been included under Home Rule anyway under this plan). The other three counties simply didn't have enough Unionists to bring them into Northern Ireland, one reason why Carson and Craig never got their "clean cut" of Ulster from Ireland.