Dayna Kearney has been found not guilty of dangerous driving causing the deaths of four of her friends in Athy, Co Kildare in January 2015.
The 23-year-old from Crossneen, Co Carlow was also cleared of driving a car which she knew, or ought to have known, was defective.
Aisling Midleton, Niamh Doyle, Gemma Nolan and Chermaine Carroll died when Ms Kearney’s car went out of control and crashed into a van.
The prosecution claimed the under inflation of at least two of her tyres caused her car to swerve into the path of the oncoming van.
The verdicts were returned after just half an hour of jury deliberations.
Shortly after Ms Kearney was acquitted, her solicitor Frank Taaffe spoke on her behalf outside Naas Circuit Court.
He said: "It has been very, very difficult - she's a lady who has had to go through counselling for the last three and a half years because of the tragic accident and a loss of four her great friends. It has been a tremendous ordeal for her.
"She's feeling relieved, but still shattered by the events of three and a half years ago."
He added: "She has expressed her sorrow to the families. They were all her friends who died in that tragic accident - she feels their loss, and she understands the great losses suffered by those families."