Coolmine Therapeutic Community supported 1,250 people during 2014, with women accounting for two-thirds of admissions.
In its 2014 annual report, the agency said there was an increase in the numbers of people with heroin as their primary problem drug.
The publication of the report comes as official figures show almost 9,900 people nationwide are on methadone.
The has also been a 16% cut in government funding since 2009, including an almost €20m fall in HSE funding for addiction services.
Coolmine, which was set up in 1973, is based in Blanchardstown, west Dublin, but most of its residential clients are from outside Dublin.
The report shows that almost seven in 10 women and half of men in residential therapy, as well as four out of 10 men on the day programme, cite heroin as their main drug of addiction.
It said that at any one time, there were 34 clients taking part in the five-month residential treatment programme at Coolmine Lodge, the male residential service. This represented a 33% increase in admissions compared to the previous year.
Occupancy also increased from 12 females to 24 at any one time in Coolmine Ashleigh House and the community and day services worked with 139 individuals during the year, up 11% on 2013.
“Coolmine’s longitudinal outcomes study found that, two years after therapy, 71% of clients were illicit drug-free; 97% did not engage in crime and 25% were engaged in employment,” said chairman Alan Connolly.
Pauline McKeown, chief executive at Coolmine, said that during the year, 64% of female clients admitted to Coolmine Ashleigh House and 50% of males in Coolmine Lodge were from outside Dublin.
She said Coolmine’s housing and resettlement service supported over 250 clients to access accommodation in that year.
“32 mothers and 21 children were homeless or inadequately housed after completing their residential programme in 2014,” said Ms McKeown.
She said there were 18 mother and child admissions and four expectant mums at Coolmine Ashleigh House and that funding had been secured to increase numbers to 24.
“In addition, the crèche facilities will be renovated to allow Coolmine to work with more children impacted by parental substance misuse on a full-time residential basis. We have also strengthened our commitment to evidence-based treatments through the Parenting under Pressure Programme for high-risk families.”