The IRA only had two weeks left of supplies on the signing of the Treaty. If the Dail refuses I imagine Britain will stay but attempt to co-opt pro-Treaty elements.
When the IRA is left without supplies, the British will probably use the division and their strength to get more from them - a devolved Southern Ireland as opposed to a dominion Free State?
The problem for Britain is that by this point the population of the South is firmly in support of independence in some form so trying to hold on to the 26 Counties by force isn't a viable option. While the IRA may be almost exhausted it will only be a matter of time before a fresh insurgency begins. However the Irish public were fed up with the War by now and probably wouldn't have supported a renewed IRA campaign so this may actually strengthen Collins and the Pro-Treaty faction.