There is a HS article here (paywalled) that states that:
Some of those closest to the accused, including former St Pat's classmates, said that the accused was often on the outer and that he struggled with his mental health.
That welfare staff at St Pat's dealt with frequent outbursts.
The accused exhibited eccentric behaviour and was sometimes bullied.
The former classmates said his mental health issues were well known.
(He likely would have been 12, 13 & 14 years old while at St Pat's. imo)
While in his first court appearance it was stated that the accused ....
did not suffer from any drug problems or mental health conditions. Link