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.

Court Orders Demolition Of 29 Unauthorised Modular Homes Built By Kildare Developer
€239k For Clane Playground As Kildare Secures Funding For Just Two Projects
Minister’s Letter: DART+ South West Entirely Dependent On Successful Delivery Of DART+ West
Kfm Weekend Obituary Notices
Two Men Charged In Connection With €4 Million Cocaine Seizure In Kildare And Wexford
Driving Testers At RSA Balloted For Industrial Action Over Concerns About Uninsured Vehicles
Homes Undelivered After Substantial Deposits Paid, Kildare Council Warns Of Potential Unauthorised Works