In the euphoria of post-agreement modern times it is tempting to think that the Irish political map has changed forever. Once the logic of having two large parties on the Southern right has been made obsolete by a new Northern deal, perhaps the old foes might shake hands and join forces in a single new party?

Forcing Fianna Fail and Fine Gael into bed with each other is, of course, the political Christmas for which the Irish left has been waiting so long. This is precisely why it will never happen.

The dole queue of unemployed TDs would be just too long and our friends in Leinster House are certainly cute enough to do what it takes to keep themselves in business.

More plausibly, the Coke and Pepsi of Irish politics will find all sorts of new ways to look different. Fine Gael will probably polish their image as liberals - more right wing on the economy, more secular, more international, and more progressive on social values.

Fianna Fail can market itself as a hybrid between the Christian Democrats and an Irish version of the Gaullists - centre-right on the economy, less anticlerical, more "Irish", and standing for more traditional social values.

Where will the end of civil war politics leave the Irish left, however? This is why recent developments in the North have once more raised the issue of a merger between Labour and DL.

Mergers between two organisations almost never take place on equal terms. One is typically more needy than the other.

In this case, DL almost certainly has most to fear from the future. How long can a party with no national vote share to speak of continue to survive on the sheer willpower of four high-profile and very able TDs with well-tended local support bases?

Individual members of the existing DL parliamentary party do thus have quite a lot to gain from merging with Labour. The current four TDs, all essentially Dublin-based, would each be more secure, having a larger pool of votes to draw from without the added hassle and insecurity of managing inter-party transfers.

Taking this narrow view, how would Labour fare? For three of the four DL seats, the Labour candidate was runner up, losing a seat in each case. It is not hard to imagine that more co-ordinated local vote management and a more focussed local campaign would reap dividends in each of these cases.

Taking a wider perspective, one view is that a single combined party of the left could hammer home a more coherent electoral message. The ideological polarisation between right and left would become more clear-cut, while economies of scale could allow a more effective use of scarce campaign resources.

However, when the prospective partners take instruction on the political reality of mixed marriages they will find almost no European example of two parties which have merged and gone on to increase their combined electoral strength. Nearly always, the new party's vote share is less than the sum of its component parts.

Sometimes the electoral system forces political odd couples into bed with each other, despite this.

In Britain, for example, a grossly disproportional electoral system does force disparate groups to stay together in a single large party for fear of otherwise disastrous electoral consequences. This is why the British parties are very broad churches.

But the same logic simply does not apply under the PR-STV electoral system, where there is no huge electoral penalty for running two smaller parties rather than one larger party with the same combined vote.

The logic of electoral campaigning, moreover, suggests that Labour and DL, operating under PR-STV, should remain separate parties. One party can set up only one ideological stall. Two parties can set up two different stalls, and between them they can cover more of the political waterfront.

Voters, of course, are better off if they can choose between different parties offering them different policies.

There is absolutely no harm for Labour and DL, therefore, if they can come to some form of tacit understanding about how to carve up the political marketplace. For better or for worse, the Labour leadership has opted for a centrist strategy, and the success of Blair in Britain has certainly beguiled the leaders of many European social democratic parties.

Labour luminaries in Ireland are not yet walking around in Armani suits, but they've certainly been window-shopping.

Not all voters live in the centre of the political world, however. There is still, as we know when we pull our heads out of the sand, massive social exclusion in modern Ireland. And even the "haves" will not always have it so good.

The current property boom may end in tears, as it did in Britain, trapping thousands with negative equity and giant mortgages. The Irish economy is overheating faster than any in Europe, and the main policy instrument to correct this has been taken away with the advent of EMU.

Rising inflation will put Partnership 2000 under serious stress and threaten the economic consensus that contributed so much to the current economic boom. If Labour's instinct is to drift to the right, then this will leave the party open to attack on its left flank.

Joe Higgins's Socialist Party has already tested the water on the left of the political beach and found it not horribly cold. Rank-and-file trade unionists are becoming less patient with leaders they see as Uncle Toms with too much of the Dublin establishment.

It thus seems clear that a DL-Labour understanding, if it is to consolidate the centre-left in Ireland, must promote a more attractive mix of hard and soft-left positions to make sure that most Irish left-wing voters have a party that they feel enthusiastic about supporting.

It is only a matter of time, furthermore, before at least one Irish party hits electoral pay dirt in the increasingly rich vein of anti-European feeling among the electorate.

The Greens and the Socialist Party offer the only anti-European alternatives at the moment. These could be the ones to benefit if the other parties fail to react to the large "no" vote on Amsterdam.

One logical feature of a tacit Labour-DL deal to sew up the left would have DL moving steadily to a more anti-EU position over the next two or three years, while leaving Labour as part of the Euro-club.

If they are to build on their current position, therefore, the two parties of the mainstream left do not need to merge, but they do need to come to some sort of understanding. A softly-spoken agreement between Labour and DL could allow them to position themselves more effectively to take advantage of the electoral mood that may develop when everything in the garden, which so many people are so proud of at the moment, begins to smell a little bit less like roses.

Michael Laver is professor of politics at Trinity College, Dublin and director of the Policy Institute. He is author of Playing Politics: the Nightmare Continues and of A New Electoral System for Ireland, to be published next week by the Policy Institute.