Some 86% of young people who participated in Youth Advocate Programmes reported a decrease in substance abuse.
The organisation's annual report shows that there were also improvements in their self-esteem and a reduction in self-harming.
The organisation worked with around 500 young people referred by child and family agency Tusla across 22 counties last year.
Siobhan O'Dwyer, CEO of YAP, explained how the programme works.
"We work with young people who have a range of issues in their lives that are referred to us by Tusla, the Child and Family Agency, and we match them with a community-based advocate who works with them to help them to achieve their goals, over six months, in the community," she said.