`Well Done, David!' These are the words which, for most people, sum up the seismic shifts which are, even now, taking place in the landscape of Irish politics.

It's true that many of the delegates at the RDS last Saturday afternoon were gobsmacked by Gerry Adams's congratulations to the Ulster Unionist leader. The Sinn Fein president moved, with all the practised skill of a society hostess, to defuse what could have been a tricky situation.

"Republicans never know whether to clap," he joked. Actually, they do. Sinn Fein activists are programmed to applaud any mention of the IRA and the prisoners, and boo the RUC, the Brits and, yes, David Trimble. Changed times indeed.

I understand the reaction was equally uncertain in the Europa Hotel in Belfast when news of Mr Adams's welcome for the unionist vote came through. Steven King, John Taylor's clever and constructive adviser, chose his words with some care on Monday night's Questions and Answers. People had been suspicious that there was an element of "malice aforethought".

There may well be the same suspicions among some Sinn Fein activists if David Trimble decides to send a "Well done, Gerry!" message, depending on what the party decides at its reconvened ardfheis.

These are outward signs of change to which, if all goes well over the next few weeks, we will all become accustomed. Like it or not - and it would be extraordinary if each man was not experiencing some problems of adjustment - David Trimble and Gerry Adams are on the same side now.

Both are committed to making the Good Friday Agreement work for the common good. They may be putting wholly contradictory spins on it - one saying it strengthens the Union, the other that it brings a united Ireland closer.

The truth is these are questions that can be answered only in the long-term future. For the moment the task is to create a new political environment which will allow the two communities in the North to become more at ease with each other and thus enable them to work together in a way that has been impossible in the past.

Gerry Adams put it well in his ardfheis speech when he said the task, for both himself and David Trimble, is to take on the challenge of "managing and planning the future". One veteran Belfast republican, a man who has played his part in - how shall I put it? - all aspects of the struggle, said to me on Saturday, "it's a bridgehead. We have to take stock from here, recognise that what both communities need now is time".

This man believes passionately that, given the changing demographic and political realities in the North, a united Ireland is inevitable. But he also accepts the task now is for both communities to learn to live together so that the future, whatever it brings, can be worked for in peace.

For that to happen, for the agreement to become so firmly bedded down that it can resist the storms and frost that will assail it, other developments have to fall into place. The most immediate priority is that the two referendums should be carried in a fashion which allows no room for doubt about the overwhelming will of the people - in either part of the island.

This is where Gerry Adams must give leadership which extends beyond the narrow concerns for his own party and takes account of the broader picture.

There is still a great deal of concern in the North, not so much about whether the vote will be carried, but about the size of the majority. From the start of this whole exercise, moderate unionists have expressed anxiety that the referendum could be passed north of the Border with only a minority of the unionist electorate in favour.

This would hand a moral victory to the Rev Ian Paisley, paving the way for a bitter and divisive Assembly election that would leave David Trimble dangerously vulnerable.

The one thing that could help this situation would be for the people of the Republic to send a strong message, through their vote, that they want to make to make the agreement work. That will happen only if Sinn Fein decides to signal its endorsement for the deal in both parts of Ireland and, at the very least, stops short of canvassing against changes to Articles 2 and 3.

It isn't so much a question of whether the referendum will be carried. Given the support of all the constitutional parties, together with that of John Hume and Seamus Mallon, the result should not be in doubt. Most voters in the Republic know the debt they owe to the SDLP for its courage in holding the line for constitutional nationalism in Northern Ireland.

But there are more than political debts to be considered here. If the North is to have the opportunity to rebuild its shattered society, then it needs to know there is a tidal wave of goodwill down here, and that the Republic is prepared to make sacrifices, like the changes to Articles 2 and 3, to speed the project on its way.

It has been suggested that Mr Adams and his colleagues may be tempted to "cherry-pick', campaigning for a Yes vote in the North, while rejecting the proposals for constitutional change in the Republic. These signals followed the strong opposition, particularly from Southern delegates, expressed at last weekend's ardfheis.

There is also the argument that this tactic could be to Sinn Fein's political advantage, attracting Fianna Fail voters who are known to be unhappy about constitutional change.

This might be cute politics in the short term, but it would raise questions about Sinn Fein's claim to have an all-Ireland strategy, not to mention Gerry Adams's pledge last weekend that "this must be a truly national struggle, not a Northern struggle with the South tagged on". Much more important for all the people living on this island, it would cast most serious doubt on Sinn Fein's commitment to making the Good Friday Agreement work.

Mr Adams has argued that this is a transitional arrangement, a new phase of the struggle for his party in achieving a united Ireland. He has said the challenge is for all Irishmen and women, himself and David Trimble included, "to join in managing and planning the future".

For that to be possible, the agreement needs all the help it can get, in the shape of popular support, in the weeks and months ahead. In particular, all those who have worked to bring the project this far, including Sinn Fein, have to demonstrate their commitment to making the new order a reality.

That means campaigning for a Yes vote - in both parts of Ireland.