Ireland would not have achieved even a tenth of the progress it has made on environmental protection were it not for the "driving force" of EU directives, according to the Minister for the Environment, Mr Cullen.
In an interview with The Irish Times on the Nice Treaty debate, he said the environment was "one of the best examples of our involvement in Europe because it has brought a lot of discipline, even a lot of pain, but ultimately an awful lot of good".
The EU had "forced us into issues, like special conservation areas, protection of the bogs and all that, which we would never have agreed to nationally ourselves. Emotionally, the arguments, the debate, would still be raging and we would have made no real progress."
Mr Cullen said Ireland had implemented 96 to 97 per cent of EU directives on the environment, transposing them into Irish law. "I doubt if we would have even 10 per cent of that - and I'm being generous - instigated ourselves if it wasn't for the European Union.
"Legitimately, you could say that if you're for the environment, you must be for Europe. Because it's certainly true, not just in terms of laws and directives and pushing and shoving us. Europe has made a huge financial commitment to helping us implement them."
Describing the Rural Environmental Protection Scheme (REPS) as "a huge environmental influence in rural Ireland", he said the EU's commitment amounted to more than €2.03 billion, not counting €230 million provided by Brussels for farm-waste management.
Since 1994, €1.1 billion had been provided for water services, of which €980 million came from the cohesion fund for less developed member-states. A further €206 million had been allocated for sewerage infrastructure, all from the cohesion fund.
The cohesion fund would also play a major role in assisting EU enlargement because it would enable countries in central and eastern Europe to tackle environmental degradation caused by dirty industries and bad practices during communism.
The Minister said EU enlargement was "a big plus", particularly in relation to ageing nuclear power plants for which the applicant countries had to sign protocols agreeing to strict monitoring standards.
During the World Summit in Johannesburg, Ireland and the other EU member-states had been centrally involved in presenting an ambitious global sustainable development agenda. A No vote on Nice would "isolate us from our fellow EU members".
Mr Cullen credited northern European countries, particularly Scandinavian ones, for their "extremely positive influence" in establishing EU best practice.
"Asked about the many complaints about Ireland's environmental record brought to Brussels by individuals and groups here, he said this reflected the fact that Europe "isn't just about big players and dominant countries, but about the individual as well".
The European Commission was "in many ways more transparent and more open than many national governments, which means that individuals have greater access to get an issue onto the European agenda to pressure governments to do more".
He criticised the anti-Nice stance adopted by the Green Party, saying it put it at odds with other European Greens, who had made it clear that there were coherent environmental, political and practical advantages in implementing EU enlargement.
"I completely disagree with Deputy Gormley that this objective would be well served by a No vote . . . This position is naïve, reckless in relation to the well-being of the applicant countries, impractical in the extreme and detrimental to Ireland's national interests."