A 23-year-old man suffered serious head injuries when the car he was travelling in was rammed on the N4 in the midlands at lunchtime yesterday.
He was being treated in Tallaght Hospital last night after being airlifted from the scene at Lamagh, near Newtownforbes, Co Longford.
His condition was described as "serious but stable."
A woman in her 50s, who is related to the hospitalised man, was arrested and questioned at Longford Garda Station last night.
Gardai are officially describing the shocking incident as a hit-and-run.
It is understood a bitter dispute had broken out between the occupants of the two vehicles that were involved in yesterday's incident.
The woman was the only occupant of her vehicle, it is understood.
The occupants of both cars are members of the Traveller community with addresses in Longford town and Edgeworthstown. Sources say gardai have been monitoring a dispute that has broken out among their associates and which has escalated in recent weeks.
"There were initially grave concerns for the welfare of the injured man because of the head injuries he suffered, but his condition has stabilised," a source said.
Less than half an hour after the ramming incident happened, there was another collision on the same stretch of road, but on the other side when a car was rear-ended.
The incident is believed to have been caused by an oil spill.
No one in the second incident suffered serious injuries.
Gardai said it was not linked to the first incident.
The road between Longford town and Newtownforbes was closed for several hours yesterday with local diversions in place as a garda forensic collision investigation was completed.
Gardai were also at the scene of a serious collision on the Swords Road, Malahide at the junction of Chamley Gardens.
The collision occurred shortly after 3pm.