The Minister for Finance was accused of attempting to change the Constitution through amendments to the Freedom of Information (FoI) Act.
Labour's finance spokeswoman Ms Joan Burton predicted court challenges and said Mr McCreevy was trying to change a section of the Constitution on collective government responsibility and accountability.
Granting power to a department secretary general to issue a certificate to prevent the release of information from another department was effectively putting the secretary general "above the Minister who is elected by the people to the Dáil", she claimed.
Ms Burton was speaking on the third day of the committee stage debate on the Freedom of Information (Amendment) Bill during which Mr McCreevy accepted a Fine Gael amendment on personal information and the "spirit" of another linked proposal.
However he refused to accept any amendments on the sweeping powers of certification for department secretaries general to restrict any information on the "deliberative process" of Cabinet.
The Fine Gael spokesman, Mr Richard Bruton, said the proposal only to give the number of certificates issued and not the topics meant "the certification issue itself is now going to be a secret".
Mr Bruton also expressed concern about a section of the legislation where all records linked to tribunals would "effectively be exempt" from disclosure. He believed there should be a "harm test", that information would be exempt where it might "prejudice" the workings of a tribunal.
Mr Bruton said there should be no question of personal records linked to a tribunal being protected from the individual concerned. If there was a case of a child being abused, the parent of that child should have access to those records. He also insisted that records relating to the general administration of inquiries should not be "closed down".
Mr Bruton said the DPP should still be obliged to account for the official administration of his office. "It is not a matter of the utmost sensitivity that threatens the security of the State."
Mr McCreevy said the amendment to tribunal information was intended to protect non-statutory as well as statutory inquiries and "any suggestion that the process is less than confidential would undermine" its effective operation. However, in line with the changes he accepted earlier on records of personal information, he would accept the Fine Gael amendment and agreed to draw up a new amendment for report stage on the general administration of inquiries.
Mr McCreevy refused to accept any changes to section 17, which restricts from public scrutiny any information linked to State security, defence and international relations. Governments with whom the State had dealings needed to be "absolutely confident" that these dealings were confidential.
Ms Burton said that gave the "green light for people with secrecy mania" to say all information was "barred" from release.
Mr Bruton said the Minister already had the power to issue certificates on important matters of State security or international relations. "If it ain't broke, don't fix it," he said.
The Green Party spokesman, Mr Dan Boyle, said the proposal was to save the "political embarrassment" of the Department of Foreign Affairs where information refused under the Freedom of Information Act had already been in the public domain.
Mr McCreevy refused to accept an amendment from Sinn Féin's finance spokesman, Mr Caoimhghín Ó Caoláin, to give access to personal records to individuals affected by sections of the Offences Against the State Act, subsequently deemed unconstitutional by the Supreme Court.
Mr Ó Caoláin said "a substantial body of people" were dismissed from or refused employment because of the Act. If they were to "pursue their legitimate right to compensation", they should have access to records and information in the hands of State departments and public bodies to help their case.
