"Greenhouse gas" (GHG) emissions from the transport sector have risen by almost 130 per cent since 1990, the biggest increase in the pollutants from any sector of the economy.
The latest figures from the Environmental Protection Agency (EPA) show a reduction in GHG emissions for the second year running.
However, Ireland's GHG output is still almost double the limit set by the Kyoto agreement.
By 2012, Ireland is committed to reducing GHG to a limit of 13 per cent above their level in 1990. Figures for 2003 show emissions at 24.7 per cent above 1990 levels.
"It's going to be difficult to reach that target; the reduction in the last two years is a step in the right direction, but the growing transport sector is going to be a challenge," an EPA spokeswoman said.
However, the Minister for the Environment, Mr Cullen, said he was confident Ireland would meet its Kyoto obligations.
"These figures are in line with recent projections used in the development of the National Allocation Plan for emissions trading, an important element of our strategy to comply with the Kyoto target."
He said Ireland was on the "Kyoto path", but it was essential that all sectors stay on target, particularly in times of economic growth.
"Ireland still faces significant challenges in meeting this target, and policies and measures will need to be implemented as current and projected economic growth act to increase emissions across the economy into the future."
Mr Cullen said he was satisfied that Ireland would "meet our international environmental obligations in an economically-efficient manner".
Agriculture is the largest GHG polluter, accounting for 28.9 per cent of emissions, followed by the energy sector at 24.5 per cent, with transport the third largest contributor at 17.7 per cent.
Emissions rose steadily during the 1990s, according to the EPA, and showed their first decrease in 2002, with 68.88 million tonnes of carbon-dioxide produced, down from 70 million tonnes in 2001.
There was a further decrease last year, with just 66.60 million tonnes emitted.
The switch in the energy sector from "carbon-intensive fuels" such as coal, peat and oil to natural gas was the primary reason for the reduction.
The closure of the IFI fertiliser plants saved 940,000 tonnes of carbondioxide in the industrial processing sector.
The EPA said in agriculture CAP reforms were helping to bring down emissions.
The bulk of GHG in agriculture come from livestock passing wind.
Methane and nitrous oxide output was reduced after "small reductions in livestock numbers".
The limits put on industry through "emissions trading" from January would also have a positive impact, the spokeswoman said.
"Basically, companies are only allowed to emit so much pollutants throughout a year, and anything above the set level they have to pay for.
"If they manage to emit less than the set level they can 'sell' the remainder, so it's a big incentive to keep GHG down."
