Nearly 8,000 incidents of criminal behaviour, involving 3,500 children over the last seven years were not dealt with properly by gardaí.
A major review of Garda Youth Diversion Programme has found that 55,000 referrals made to it were deemed not suitable.
Of those, 7894 referrals - involving 3,489 under-18s - were not properly progressed, meaning they should have been charged or summonsed.
Those incidents include public order, theft, and traffic offences, but also 55 serious crimes such as cases of rape, sexual offences and child neglect.
This error led to around 3,500 people and businesses being affected by crime, but no prosecution occurring - even though gardaí had identified a suspect.
Gardaí say the issue arose due to a lack of awareness and training.
They’ve now been set up with an e-learning system, and the Policing Authority will quiz the Garda Commissioner further this afternoon.
Letters apologising to victims are being sent out and a helpline has been set up.
The number is 1800 589 589, and gardaí will also visit some of those affected.
Garda Commissioner Drew Harris said: "I want to apologise to the more than 3000 individual and organisational victims that we have let down.
"They expected us, An Garda Síochana, to pursue the crimes committed against them, to fulfill to the fullest extent possible our responsibilities in terms of investigation.
"In their cases, we did not do this - and I say again, I profoundly apologise."
