There would be little point in attempting an invasion as they were deterred from carrying out operation Sea Lion. Invading Eire would have involved greater distances in an area where Britain would have had naval supremacy and air cover would have been limited. It would have been in Britain's interests for any attempt to have been made as the ionvaders could have been cut off and De Valera would almost certainly have had to request assistance at which stage Britain would be able to demand the use of the 3 treaty ports. The Irish American lobby would have turned from neutrality to hostility to Germany
Britain considered siezing the 3 treaty ports but it was regarded as more trouble than it was worth as Eire was supplying volunteers for the armed forces many of whom would have resisted a British invasion. Collaborators would have been fairly few, possibly from the extermes. O Duffy's blue shirts who went to Spain to fight for Franco and the IRA who fought for the Republicans but opporturnistically worked with Nazi Germany in 1939. De Valera would have either tried to have been a Churchill or more probably would have been a spent force having not exactly been prominent in the fight for Independence unlike Collins and whose opposition to the treaty was more political posturing than armed resistance. As it was more people from Eire enlisted than from Northern Ireland, an invasion would have increased the ranks of the allies. Eire would probably have joined NATO and would have been in the UN earlier as the Soviet Union would have no excuse for a veto