Well, assuming that the disastrous execution of republican ringleaders after the Rising still goes ahead, Britain is still certain to have decided Ireland must have home rule sooner rather than later. With Collins removed, and assuming no-one else of military competence rises to the fore, Irish independence will still occur, perhaps a year or two later and without the civil war that followed afterwards. I wouldn't be too surprised if De Valera accepted the same 'Irish Free State' deal that OTL he derided Collins for. Without the prolonged fighting, he may not be so eager to leave the commonwealth completely, and Britain may be able to hold onto the Treaty Ports at least until WW2, which would make things much easier in the Battle of the Atlantic.