One shot was fired by a man during an armed raid at a service station on the M9 at Junction 5 in Co Carlow on Wednesday morning.
A Garda spokesman said two men, one armed with a gun and another with a knife, entered the station at Tinryland just after 2.30am.
One shot was fired during the robbery, and both men then took a small amount of cash and drove off in a dark coloured Volkswagen Golf. Gardaí said no one was injured during the incident.
In a follow up operation later that morning about 9am, gardaí from Carlow Garda station helped by the Armed Support Unit carried out a search at a house in Carlow town.
Three men and one woman, all aged in their 20s, were arrested and are in custody at Carlow Garda station.
Gardaí are appealing for anyone with information to contact Carlow Garda Station on 059 913 66220 or the Garda Confidential Line on 1800 666 111
