The Dáil has voted to support the Government's policy of continuing to grant overflight and landing rights in Ireland to US military and civilian aircraft.
The motion was carried by 77 votes to 60.
Earlier, the Taoiseach and the Tánaiste strongly defended the continued use of Shannon by the United States military.
Bertie Ahern told the Dáil that to withdraw facilities for American forces at Shannon Airport now would be a radical change in Irish foreign policy and would be seen as a hostile act.
He said it would be counter to Irish interests and weakening the close ties between Ireland the US and the UK.
Tánaiste Mary Harney agreed, describing the US as our closest friends, with ties that run deep.
The Taoiseach said that Ireland recognised that the US and the UK believed they have a mandate from existing UN resolutions, to wage war on Iraq.
However, Mr Ahern said that Ireland had made it clear that it required a further UN resolution.
'Creative' view of UN resolutions - Rabbitte
Earlier the Labour leader Pat Rabbitte accused the Government of 'creative interpretation' of UN resolutions to justify US facilities at Shannon.
Mr Rabbitte said no-one seemed to know the actual reasons for the war: liberation, disarmament, a terrorist threat or commercial interest.
The Green party and Sinn Féin also criticised Irish support for the war.
FG leader questions legitimacy of war
The Fine Gael leader Enda Kenny told the Dáil that the Government could not uphold the legitimacy and primacy of the UN and allow the use of Shannon Airport.
He said resolution 1441 did not authorise war as every other possibility had not been tried.
Saying the war was of doubtful legitimacy, Mr Kenny added that Ireland was facing a critical moment in our diplomatic history.
He said it is up to this country to say either we believe in the legitimacy of the UN or we do not; Or we agree to be bound by the decisions of that organisation or we do not.
The motion the Government has put before the Dáil runs to 16 clauses. The most controversial aspects of these relate to the final two clauses, which recall the longstanding arrangements for overflight and landing of US aircraft in Ireland, and support the Government's decision to maintain those arrangments.
The Taoiseach has claimed that there is clear legal support for the view that their provision does not amount to active participation in a war.
Fianna Fáil backbencher Sean Haughey said the ‘moral thing to do’ would be to withdraw landing facilities at Shannon, and he regrets that it is not possible to do so.
Deputy Haughey said he must reluctantly accept the Government's view that it was in the national interest to continue providing the facilities.
PD Fiona O'Malley said she was ‘uneasy’ about the continued use of Shannon by allied aircraft in the absence of a second resolution.
However, she said she accepted the advice presented by the Taoiseach to the House that the continued use does not breach our Constitution.
All of the Opposition groups have put down their own amendments to the motion.
Fine Gael are pressing an amendment opposing participation in any manner, or support for the war.
Labour's alternative will deplore the decision to provide facilities at Shannon and call on the Govenrment to end all cooperation with those operating outside the UN Charter.
Sinn Féin is to call for an end to overflight and landing rights for the duration of the war.
Attacking the Government motion as 'spineless and craven', the Green Party called on Fianna Fáil and especially PD backbenchers to break ranks and vote against the Coalition.
However, Government sources appeared confident that all of their TDs will support the motion.