One-in-five teenagers seen by a leading drug treatment centre are hooked on substances bought in head shops.
Two-thirds of head shops have closed, at least temporarily, in recent days after the ban on drugs such as mephedrone, BZP and synthetic cannabinoids.
But as many as 20pc of youngsters being treated by a leading drugs psychiatrist now list head-shop drugs as their "primary addiction".
This raises fears of a massive underground market in the substances waiting to be tapped by organised criminals.
Consultant child and adolescent psychiatrist Dr Bobby Smyth said that some of the recently banned substances were actually more dangerous than long-established "illegal drugs".
Dr Smyth works with the National Drug Treatment Centre Board in Dublin, one of the country's largest organisations working in treating addicts. According to him, between 150 and 200 teens come to him for help with their drug problems every year.
Dr Smyth said: "I work especially with under-18s in drug and alcohol treatment and I'd say that those with addictions to head-shop drugs make up about 20pc of the people I come across now.
"This number has steadily grown since early 2009, but it should be said only the minority of this amount have head-shop drugs as their primary addiction -- often it's associated with alcohol or other hardcore drugs.
"Cannabis, I would say, is a safer drug than the ones replicating it that were sold in the head shops."
But he added that "overall the legislation is a step in the right direction. It's hard to bring in changes that won't have some downside, but it's worth it".
The legislation had an immediate impact on the 100 head shops operating countrywide, with the Government and campaigners agreeing that around two-thirds had shut their doors.
But some retailers have already started to offer alternatives to mephedrone and other banned substances.
Meanwhile, the now illegal products can still be bought over the internet from Asia-based sellers.
Grainne Kenny, Irish president of Europe Against Drugs, said head shops were "going down like a house of cards".
She said those that remain open were now mainly selling drug paraphernalia such as pipes, bongs and growing equipment. These items will be banned under the new Criminal Law (Psychoactive Substances) Bill which is due to be passed by the Dail before the summer recess.
"As long as they have the paraphernalia there for people who shop on the internet, they have the means to use what they buy. Once it is banned, the head shops will be gone," she said.
At one stage it was claimed that Dublin had the highest concentration of head shops in Europe with 21 stores in the north and south inner city.
Mel Mac Giobuin, of Dublin's North Inner City Drugs Taskforce, said it was still too early to say if the closures were for good.
"It's not clear to us if people are closing for the day or if it is long term.
"There is some speculation that there may be other products not covered by the extensive ban.
"A number of different compounds and different families of drugs have already started to be used in the UK since the ban on mephedrone there last month."
Breda Heffernan and Michael McHale
May 17, 2010