OK heres a point of departure.
Michael Collins isnt assassinated by anti-Treaty forces.
de Valera is thus a minor figure, doesnt become President of the Council, and there is no Anglo-Irish trade war)
As a result, the UK keeps the Treaty Ports. (Indeed, during the negotiations Collins saw how vital it was, not just for Britain, but Ireland as well, to keep the atlantic approaches well defended in time of war).
The Irish Free State, without Dev, while not as belligerent towards the UK as in OTL, still declares neutrality, but the presence of the Treaty Ports means it suffers from German bombing, and it declares war in 1941.