The Taoiseach succeeded last night in deleting a reference to tax harmonisation from the new EU treaty on the grounds it could threaten Ireland's low business taxation regime.
Mr Ahern and the French Prime Minister, Mr Lionel Jospin, had expressed sharp differences on the matter throughout yesterday's EU summit. Discussions continued on the matter until a breakthrough shortly before 9 p.m.
Mr Ahern and the British Prime Minister, Mr Blair, last night successfully persuaded their EU colleagues that political commitment they made at home made it impossible for them to accept any reference to tax harmonisation in the treaty.
Other key Irish interests under threat last night included the automatic right to have an Irish member of the European Commission, although a reasonably attractive compromise appeared to be on offer.
The Government succeeded at the weekend in protecting Ireland's social partnership arrangements from becoming subject to EU majority voting. Ireland will continue to retain the right to organise its social partnership arrangements as it sees fit following a lengthy debate on Saturday.
Other changes affecting Ireland taking shape last night included a reduction in our number of MEPs to 11. Ireland's voting "weight" at Council of Ministers level seems set to be fixed at seven - equivalent to that of Finland, Denmark and the applicant Slovakia. The Government was relatively unconcerned about the issue of its voting weight. According to one source: "With 337 votes in the Council, it makes very little difference whether we have seven or 10."
However on the key issue of taxation, Ireland insisted the French draft reference must be dropped. Mr Blair led the opposition to the clause while Mr Ahern made several interventions on the issue.
Earlier senior Government figures said the section on tax harmonisation "must go" if Ireland was to agree to a deal at Nice. "The argument for qualified majority voting in this area is not a good argument and I would have to say again the people who put the argument here have done nothing to impress me," Mr Ahern said yesterday.
Ireland was determined to protect its low business tax regime on the grounds that it has attracted substantial foreign investment. The French EU Presidency was equally determined to include the clause - described throughout as "the thin end of the wedge" by senior Irish figures. The text now agreed retains the national veto on taxation issues. However the council last night deleted a clause saying that after five years a summit could unanimously decide to move to majority voting on aspects of indirect and company taxation.
Ireland resisted including even such an aspirational clause. "Our advice is that there is a risk involved," a Government spokesman said. "We are not accepting it. It is formalising their right to bring forward proposals."
Acknowledging his differences with Mr Jospin, Mr Ahern said yesterday: "Lionel Jospin has a point of view. He and I have crossed swords on this. He doesn't agree with me and I don't agree with him and I'm not going to change my mind during Nice."
Ireland appears to have accepted a formula that would end our automatic right to nominate a EU Commissioner once the EU grows from its present 15 members to 27. At that stage, according to the compromise formula, the European Council will set a maximum number of Commissioners which "shall be less than the number of member states".
Ireland and nine other smaller EU member states agreed among themselves to accept this proposal, on the basis that Commission membership will be strictly rotated with the larger states given no advantage over the smaller ones.