THEY called him the Prince of Darkness. For most of his five years as head of the Anglo-Irish division at the Department of Foreign Affairs, the North's unionists portrayed Sean O hUiginn as a green bogeyman.

But now that he is widely expected to move to Washington as ambassador later this year, unionists would be mistaken to suppose there will be a change in policy emanating from Iveagh House.

For his replacement is widely expected to be Dermot Gallagher, present Ambassador to Washington. Mr Gallagher is coming home in more ways than one he held, the Anglo-Irish post before going to Washington and being succeeded by Mr O hUiginn.

If unionists saw Mr O hUiginn as "green" during the period of the Downing Street Declaration, the Framework document and the lengthy decommissioning saga, they will have seen Mr Gallagher as the same during the unsuccessful three-stranded talks of 1991-92. The two men differ in style, but there is no apparent difference in determination to stick to the line.

The Department insists no decision has been made formally, and that diplomatic reshuffling is a matter for the Tanaiste and the Government. But these two men are very likely to swap places.

During each of their periods heading the Anglo-Irish division, Iveagh House stuck doggedly to Government policy. During Mr Gallagher's time, the issue for the Government was that of pressurising unionists to talk to the Government here after inter-party talks in the North.

After months of negotiation, North/South talks finally took place. Under political direction, the Department of Foreign Affairs and Mr Gallagher were seen as the cutting edge of a tough Dublin negotiating stance.

In Mr O hUiginn's time, the issue was the content of the Downing Street Declaration and the Framework document, and his intellectual input into negotiations and the drafting of both was regarded as critical. On a number of occasions the Department of Foreign Affairs successfully resisted British and unionist pressure - as well as indications of softening from the Department of the Taoiseach - to compromise on issues they regarded as crucial.

"O hUiginn held the line against some of the waverers in Government Buildings during these last few years," says an admittedly partisan senior Fianna Fail source. He added that his party would be equally happy with Mr Gallagher in the key post.

During the period of the IRA ceasefire, Mr O hUiginn developed considerable rapport with senior Sinn Fein figures. "He is an absolutely straight dealer," says one senior source. Perhaps the most serious challenge facing his successor is to develop a similar rapport should republicans edge back towards peace.

A sophisticated, urbane and intellectual official, Mr O hUiginn has been a backroom force rather than a front man. His demeanour is that of a quiet-spoken thoughtful gentleman rather than a tub-thumping prince of darkness.

Mr John Taylor of the UUP has a different view of him. "The Department of Foreign Affairs is very much disliked by unionists," he says, "and we would pinpoint Sean O hUiginn and Dick Spring as being primarily responsible for the green nature of the Department." He does not suggest he will have a different view of his likely successor. "I have met Dermot Gallagher in Washington, but I can't really express an opinion."

But while Mr O hUiginn is seen as a master conceptualiser and drafter of complex documents, Mr Gallagher's strength is said to be his ability to develop relationships with contacts.

A number of unionists still fondly recall a very pleasant evening in Mr Gallagher's ambassadorial residence in Washington during an investment conference. He has developed formal and social contacts with a wide range of senior and influential figures in the US.

In Washington, Mr O hUiginn will see his task as maintaining and reinforcing President Clinton's personal engagement with the peace process. One of the key developments during Mr Clinton's first term was the heightening of White House involvement in Northern Ireland issues, and the subsequent diminishing of the influence of the State Department, generally regarded as more open to British persuasion.

This shift was most striking in the decision to grant a US visa to Mr Gerry Adams - and separately to Mr Joe Cahill of Sinn Fein - against the wishes of the British government and the State Department.

NOW the key officials in President Clinton's National Security Council - Mr Anthony Lake and Ms Nancy Soderberg - have gone and there are new officials to cultivate. In addition there is the constant need to lobby effectively the State Department, which may regain primacy in US Northern Ireland policy.

Meanwhile, the new head of the Anglo-Irish division inherits a peace process which looks far less hopeful than two years ago. The single development that could most transform the post would be an IRA ceasefire, following which the Government and its officials would be pressing again for an inclusive talks process.

The new Anglo-Irish head will also be coming in as a new British Labour government is expected. His first few months may well be dominated by the building of a relationship with a new set of politicians in the Northern Ireland Office and Downing Street.

As the plenary session of the talks at Stormont wound up last Wednesday, Mr John Taylor stood up to raise a final matter. "I have seen," he said, "in the southern Irish press that the Government of that country has appointed Mr John O'Higgins as Ambassador to Washington. I think it is only right that we should congratulate him and wish him God speed out of Northern Ireland." According to Mr Taylor all present, including Mr O hUiginn laughed heartily.

But one seasoned London observer said: "Dermot Gallagher will soon become the new dark prince".