Father Paul Murphy, 52, was stabbed seven times with a hunting knife as he entered a barracks near the western town of Galway in his car on August 15 last year.

The teenager, who was 16 at the time, was overcome by soldiers.

He had gone to the barracks "with the intention of killing a member of the Defence Forces", Judge Paul McDermott said.

The teen, whose identity has not been released due to his age, pleaded guilty to attempted murder in February, saying he was "genuinely sorry" and could not remember the attack.

In addition to the eight-year term, he was handed a two-year suspended sentence and ordered to take part in deradicalisation programmes.

During the hearing, Murphy forgave the youth and embraced him. Murphy had done the same on Tuesday when the judge pronounced his decision.

"This young boy is going to prison for a few years. My only hope and prayer is that he will return to society better able to manage his own life and also to make a more positive contribution," said the chaplain.

Murphy underwent surgery and partially lost feeling in his wounded arms.

"Vulnerable and highly impressionable and otherwise intelligent teenagers can be the object of dangerous, manipulative, false and malicious propaganda by terrorists... throughout the internet," the judge said.

The teen, diagnosed with autism, had no prior convictions.

He adopted Islam at the age of 15, but extremist content online encouraged his "distorted thinking", the judge said.

The judge said individuals "gained his trust", leading the teen to have "intrusive thoughts" of killing an Irish soldier as a form of protest.

They conveyed "a distorted view" of the Irish forces' involvement abroad, particularly in Mali, which led the youth to devise the plan to kill a soldier.

In addition to the online content he viewed, an Islamic State flag was found among his belongings.