At a brief arraignment at the Central Criminal Court, the teenager replied guilty to the count of attempted murder at Dún Uí Mhaoilíosa, Renmore Barracks in Galway on 15 August 2024.
The defendant was 16 at the time of the attack and cannot be identified due to his age.
Mr Justice Paul McDermott requested a probation report from Oberstown Detention Centre and adjourned the matter to 7 March, when a date for a sentencing hearing will be set.
Fr Murphy, aged in his 50s, will be invited to make a victim impact statement to the court.
The victim, a popular chaplain with the 1st Infantry Battalion, suffered multiple stab wounds in the attack while he sat in his car.
He was treated for his injuries at University Hospital Galway.