Former taoiseach Mr John Bruton will make improving relations between the EU and US his primary task following his appointment yesterday as the EU's ambassador to Washington.
The Commission's announcement in Brussels drew Mr Bruton unanimous praise and good wishes from across the political spectrum in Ireland.
The Taoiseach, Mr Ahern, said the former Fine Gael leader had given "outstanding service to his country as taoiseach and as a minister", and had played "a pivotal role in developing Ireland's relations with the European Union".
Fine Gael leader Mr Enda Kenny said the nomination of Mr Bruton was "a huge honour for John personally and for Ireland. He is eminently qualified for this appointment, having demonstrated a very strong commitment to European integration throughout his career."
Labour Party leader Mr Pat Rabbitte said Mr Bruton had "resided over a period of great economic progress and social reform".
"The skills he so ably demonstrated in facilitating the work of a three-party coalition will, no doubt, stand to him in his future post," said Mr Rabbitte, who served with him in the 1994-97 government.
The Tánaiste, Ms Harney, said: "I have long been of the view that our unique historic and cultural ties with the US, allied to our active participation within the European Union, leaves us ideally placed to carry out such an important role."
Green Party leader Mr Trevor Sargent, who also congratulated Mr Bruton, said relations between EU member-states and the US "will be critical if a consensus is to be reached on how problems of world poverty, depletion of oil, climate change and war-mongering are to be solved".
Mr Bruton will join the Commission in November, and will take up his post as soon as his accreditation is approved by the US authorities and EU member-states have been consulted.
"The decision reflects the joint wish of the Commission and the member-states to raise the profile of the EU in the US," the Commission said in a statement.
The appointment follows a year of lobbying on Mr Bruton's behalf by the External Affairs Commissioner, Mr Chris Patten, who hopes the former taoiseach will draw on Ireland's experience in building contacts on Capitol Hill.
Mr Bruton said US lawmakers and the American people needed to know more about the EU and to understand better the scope for political and economic co-operation.
"There is a lack of understanding of what the EU is or an unjustified suspicion. These concerns need to be removed by persistent, persuasive dialogue."
He expressed the hope that other EU countries could benefit from Ireland's example in exploiting their ethnic communities in the US to gain access to lawmakers.
"I think the Irish were among the first countries to realise that power resides just as much on Capitol Hill [the US House of Representatives and Senate] as within the administration," he said.
He acknowledged that the transatlantic relationship had endured a difficult period, not least because of disagreements over the war in Iraq, but said that both sides were engaged in a process of reorientation following the collapse of communism.
"It does face a certain strain. The corset provided by the Cold War is gone, and that is a somewhat disorientating experience. We are going through a process here," he said.