The Green Party will decide in March its stance on the referendum on the European constitution following a major internal debate.
Over 200 Greens from 29 European countries will gather in Dublin this weekend to discuss both the constitution and the need for more common European-wide campaigns.
Emphasising that the Irish Greens were "a pro-European party", the party's leader, Mr Trevor Sargent, said he "wanted" to support the constitution.
"However, I am finding it difficult because of the impediments that are still contained within it, particularly on defence matters.
"The issue for us all is whether the pluses outweigh the minuses. We appreciate that many of our fellow Greens throughout Europe have similar concerns."
His Dáil colleague, Mr John Gormley, said: "We have yet to make up our minds. We will have regional debates. We hope to come to a conclusion in March.
"One issue that is of concern is the way in which neo-liberal values will be enshrined in the constitution."
If the constitution is accepted by all EU member-states, EU states will be required to "undertake progressively to improve their military capabilities".
In the 2005 Estimates, published last week, the Department of Defence is scheduled to spend €738 million on military pay, pensions, equipment, etc.
Mr Gormley said: "Currently, we spend 0.6 per cent of our gross domestic product (GDP) on defence. This would have to rise to 1.0 per cent.
"Others say that spending would not have to rise. I can't read it any other way, however. If we are to be part of the European Union's 'structured co-operation' then defence spending will have to rise."
Using Mr Gormley's figures, defence spending in the Republic would have to rise to €1.4 billion annually since the State's GDP this year is some €144 billion.
Mr Sargent said eight Green parties in Europe have now had experience of government, most significantly in Germany.
The European Greens' general secretary, Mr Arnold Cassola, said national parties have already worked together on food safety, climate change, renewable energy and other issues.
Calling for greater numbers of women in politics, Ms Grazia Francescato, the honorary president of the Italian Greens, said just 15 per cent of all European MPs were women.