The decline in Overseas Development Aid (ODA) in the 1990s is "shameful, indefensible and inconsistent with the commitments given at Rio", the Taoiseach, Mr Ahern, told the World Summit on Sustainable Development in Johannesburg this afternoon.
Mr Ahern also reiterated the Government's "absolute commitment to achieving by 2007, the UN target of 0.7 per cent of GNP on ODA".
He stressed that agreements reached at the world summit, many of which have been condemned by Non-Governmental Organisations, do matter and can make a vital difference.
He pushed for international official development aid to be uncoupled from national commercial interests - as in the case of aid from Ireland.
The Government was criticied earlier this year when it was revealed there would be a reduction in its overseas aid of over €32 million.
Without referring to the recent reduction, Mr Ahern said spending on aid was still increasing, in money terms with an extra €100 million is available this year.
He added that Ireland supports, in principle, the cancellation of third world debt. "We see debt cancellation as an important contribution by donors in support of the New Partnership for Africa's Development and its commitments to democracy, the rule of law, the fight against corruption and the protection of human rights".
Mr Ahern said recurring serious food crises and the unchecked spread of deadly infectious diseases "call into question the progress on sustainable development since Rio".
"An environment that is compromised affects us all. But the poor are the most vulnerable and least equipped to adapt to environmental change", he added.
Ireland was preparing to meet its Kyoto commitments and new, tougher measures required to deal with climate change, he said.