Starting in 2002, the Isle of Man began to enter into international agreements under specific, delegated, authority from the UK (Agreement Between the Government of the Isle of Man and Government of the United states of America for the Exchange of Information Relating to Taxes, 3 October 2002)
I am aware of only one pair of linked twentieth century examples of the Isle of Man being given authority by the UK to enter into negotiations on its own behalf In 1952 the Irish Republic and the Isle of Man had entered into a reciprocal agreement in relation to social security. The Manx had accepted a single document for this agreement, which referred to “Ireland” throughout. The UK policy was to use twin texts, the British text referring to “Eire”, in order to recognise the position of Northern Ireland.
This reference to “Ireland”, in an international agreement made by the UK, was controversial at the time. The UK government took a low view of the Manx side of negotiations, privately describing the process as one in which the Manx had “been hoodwinked” by the Irish. When the agreement came to be renewed in 1954, the Manx government were unwilling to do this, instead wishing a single document referring to the “Republic of Ireland”. Morley of the Commonwealth Relations Office, although accepting that this insistence was contrary to the UK practice, explained to the Irish Authorities that the Governor had responsibility for this area, with the advice and consent of a Board of Tynwald, and that he had decided to hold to the decision.
Peter Edge's Law Blog. 