The bus driver, also in his 30s, was uninjured; two passengers have non-life-threatening injuries.
Gardaí are investigating a fatal collision on the N7 eastbound at Kill.
It happened around 3.30 this afternoon between Junctions 8 and 7, involving a bus and a pedestrian.
A man in his 30s was taken to Tallaght University Hospital and has since died.
The bus driver, also in his 30s, was uninjured; two passengers have non-life-threatening injuries.
That stretch of the N7 is closed for a forensic examination and will remain shut overnight, with diversions in place.
Witnesses or drivers with dash-cam are asked to contact Naas Garda Station on (045) 884 300 or the Garda Confidential Line on 1800 666 111.
Separately, a man has been killed in a crash in Carlow.
The pedestrian in his 40s was hit by a car on the R703 in Ballyroughan Big in Borris at half 4 this afternoon.
He was pronounced dead at the scene, while the driver of the car, a woman in her 60s, was uninjured.
The Coroner has been notified and a post mortem examination will be carried out.
The road will remain closed overnight and traffic diversions are in place.
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