Over the last 15 years, enlargement of the European Union has been talked about so often that we can run the risk of taking it for granted.

In a modern world where people use the work "historic" far too readily, there is no doubt that this is a truly historic development. We should celebrate and enjoy it. But we should equally remember the extra responsibilities it places on the Union's leaders.

The European Union is a unique endeavour in world history. Its legal basis and the dynamic between its institutions is difficult to explain in simple language. Therefore, it is easy to misrepresent.

In my view the debate on whether the European Union is primarily "intergovernmental" or "federal" misses the point. The Union is a system created for a particular set of circumstances; it is not and does not seek to be like a "nation state".

Proof of the Union's success is most clearly demonstrated by the fact that it will shortly have over four times the number of member states it had at the beginning.

Particularly in an age of globalised trade and media, sharing sovereignty in agreed areas does not represent a loss. In fact, it is quite the opposite. It represents a real and tangible gain for states by giving them an influence they otherwise could not hope for.

As Prime Minister of one of the Union's member states, I very firmly believe that the Union helped us to achieve and maintain real sovereignty. Some people look at the fact that we sometimes have to implement policies we don't like and claim that this infringes our sovereignty. These people are quite simply wrong.

Membership of the European Union has in fact enabled Ireland to achieve full sovereignty. Our people are no longer compelled to emigrate to find employment. We are a successful, self-confident and outward-looking people. Our sense of ourselves as independent and equal has grown. For me this is a real and concrete expression of increased sovereignty.

This is where the real strength of the European Union lies. The spirit of tolerance, the respect for difference, the knowledge that we are all working as equals for all our people is what drives us on. The idea of winners and losers, big versus small, is not what the Union is about.

Equally, we must never be complacent. Collectively and individually we must focus on what needs to be done to ensure that the European Union continues to succeed.

The Union must continue to adapt, change and react in a creative way to the new circumstances in which it finds itself. We have to continue to work for a system of governance appropriate to a complex and changing world. A system that is responsive to the realities which we face.

It is perfectly reasonable to try to make sure that these new needs and circumstances are reflected in changes to the basic law of the Union. This is why it is so important that the Union agrees the new Constitutional Treaty at the earliest possible opportunity.

Last month the European Council meeting decided that we would complete the Intergovernmental Conference by our next meeting in June. Formal political-level negotiations will resume shortly and I have asked that no new issues be raised in the negotiations. I will also seek a spirit of compromise and flexibility from all the participants so that we can achieve our agreed objective by the June deadline.

I am hopeful that we will reach agreement on a Constitutional Treaty. I firmly believe that it will improve the basis for democratic discourse in the Union. It will be an organic development of the Union rather than a revolutionary departure - but it will be no less important for this because it will make our work more accessible and effective.

While we have to complete this work, we also have to renew our commitment to addressing a growing tendency to take the Union for granted and to dismiss its relevance. European debates are seen as detached and elitist and the Union has too often fallen victim of shallow stereotypes and the search for an easy headline. The Union can rarely expect to have the political immediacy of national and local issues. As a result it can be presented as distant.

This places a particular onus clearly on politicians and leaders. In our work we must remain respectful of the genuine differences that we will have from time to time and work to achieve consensus when possible. At the same time, we must speak in a language that our people, who are not engaged in the detail, can understand.

We must also ensure that our engagement with the Union is not diminished. There is a very genuine concern that with more players the opportunities for misunderstandings will increase. We must not let this happen.

We have to keep reminding ourselves that we are a community with overriding shared interests. Each member state must strongly promote its own position. But we should not seek out ways to highlight our dissent. Sometimes we can promote our interests in close concert with others but irrespective of the circumstances there is an obligation on each of us to ensure that debates are framed constructively and respectfully. This has been the method which has always worked best from the beginning of the European Union. It must remain our way of doing business.

Everyone in our family of 25 will have to ensure that the grounds for misunderstanding are minimised. Engagement with each other and with the people in the institutions of the Union will have to increase. This will be necessary at all levels in our administrations and across the political spectrum.

There is also a duty on people who want the Union to be effective to be public advocates on its behalf. It is often hard for moderates to be passionate and activist. However, there is a real danger that this can leave the field wide open to those who want to damage the Union. Moderates must not allow the truth to become distorted.

Enlargement will bring with it a range of new opportunities for all members of the Union - but the challenge now is to move from the blueprint to the harder task of achieving its full potential for our citizens.

There is no doubt in my mind that the enlargement of the Union will require all of us to work harder. At the same time I believe that the quality of what we will deliver for our people will depend on the level of our engagement. At every level in our system, engagement with each other in this larger, more diverse, Union will be critical to our own success and to the success of the European Union for generations to come.

The European Union has proven its ability to help states large and small, with different cultures and democratic traditions, to work together for the benefit of all. Building on this in the years to come will require us to avoid being complacent, to accept the need to engage each other and our citizens constructively.

I am confident that this enlargement will be an outstanding success due to the level of goodwill and engagement, which currently exists among the people and states across the continent. I am confident that the spirit summed up in the proverb that you should protect yourself not by force but by your friends will make sure that we remember May 1, 2004 as the date when we, as Europeans, achieved a milestone in Europe's history and a starting point for a new era of peace and prosperity throughout our 25 countries.

Mr Ahern is Prime Minister of Ireland.