The man is currently being held at Dundalk Garda Station.
A man in his 20s has been arrested in relation to a two car collision which killed three people on the N1 at Carrickarnan, Ravensdale north of Dundalk.
He is currently detained at Dundalk Garda Station under Section 4 of the Criminal Justice Act.
The occupants of one of the cars, Mary Faxton, who was in her 80s, and her son Kevin Faxton, in his 50s, were pronounced dead at the scene.
They were from Bessbrook near Newry.
A man in his 20s travelling in the second car was taken by ambulance to Our Lady of Lourdes Hospital with non-life-threatening injuries, while the second occupant of the car, Bryan Magill, who was in his 20s, was pronounced dead at the scene.
The crash site and cars involved were examined by Garda forensic collision investigators and local scenes of crime officers.
Officers have appealed for people who were on the road between 2am and 2.30am, or may have dash cam footage from the area to contact Dundalk Garda station.
Seven people have been killed on Irish roads this weekend following accidents in Galway, Louth, Monaghan and Cavan.
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