The incident took place along a stretch of the main N4 just outside Ballinalack in county Westmeath just before 2pm.
Gardaí, together with emergency services, tended to the injured duo and swiftly closed off the road to oncoming traffic.
Ambulance personnel transferred the man, who gardaí have confirmed had been at the wheel of the vehicle caught up in the crash, to the Mater Misericordiae University Hospital.
However, due to the seriousness of his injuries, a decision has this evening been taken to transfer him to Dublin’s Beaumont Hospital, where he remains in a serious condition.
The female passenger in the car was subsequently taken to Tullamore’s Midland Regional Hospital for treatment to what have been described as “non-life threatening” injuries.
The road itself remains closed this evening to allow garda forensic collision investigators carry out a technical examination of the scene.
As part of those inquiries, gardaí have launched a public appeal for information for any motorists and road users who may have come across the incident or be in the possession of potentially vital dash-cam footage to come forward.
“Gardaí and emergency services attended the scene of a serious road traffic collision in Co. Westmeath this afternoon, Wednesday 10th December 2025,” said a garda spokesperson.
“The collision involving a truck and a car occurred on the N4 at Ballinalack shortly after 1:30pm.
“The driver of the car, a male in his 40s, was brought by air ambulance to the Mater Misericordiae University Hospital and has since been transferred to Beaumont Hospital, where he remains in a serious condition.
A female passenger in the car, aged in her 50s, was brought to the Midland Regional Hospital in Tullamore for treatment of non-life-threatening injuries.
“The road is currently closed pending a technical examination by Garda Forensic Collision Investigators and diversions are in place.
Those measures have seen westbound traffic on the N4 being diverted via Castlepollard while eastbound traffic is being diverted at Edgeworthstown via Ballymahon.
“Any road users who may have camera footage (including dash-cam) and were travelling on the N4 between Edgeworthstown and Mullingar between 1:15pm and 1:45pm on Wednesday 10th December 2025, are asked to make it available to investigating gardaí,” added the spokesperson.
Anyone with information is asked to contact Mullingar Garda Station on 044 938 4000, the Garda Confidential Line on 1800 666 111, or any Garda Station.