Four members of a gang were arrested after they put a gambler under surveillance and robbed €5,000 of his winnings after he left a casino.
The victim is recovering after he was rammed off the M20 motorway outside Limerick city.
Gardai believe that the gang had the Pakistani man under surveillance in recent days before they struck.
They followed the man after he left Fitzpatrick's Casino in Limerick city with an estimated €5,000 shortly after 1.30am on Wednesday.
As the man was driving in the Cork direction along the southern ring road, he was rammed from behind by the thieves and his vehicle came to a halt.
He was threatened with a knife, beaten and robbed of an estimated €5,000 before the gang attempted to flee.
The victim was brought to hospital but has since been discharged.
Four people, two men and two women, were arrested on Wednesday afternoon in follow-up searches by gardai.
The two men, aged 25 and 31, are still being questioned under the Criminal Justice Act.
The two women, aged 28 and 31, were taken to Henry Street garda station. One woman remains in custody and the other was released yesterday afternoon.
Last night gardai arrested a man and woman both in their 20s, bringing to five the number in custody.