Narcos Dublin: Ex-undercover officer reveals 'terrifying' violence he faced - and the secret he kept from family

Being bitten by drug dealers and stabbed with syringes "went with the territory" for undercover police officer Michael O'Sullivan.

Aged just 22, Michael became part of a secret unit in Ireland's national police force when Dublin was in the grips of its first heroin epidemic in the early 1980s.

As the problem "mushroomed", the city became a "dangerous, crime-ridden area" - and it was "disastrous for law enforcement", Michael says.

"The situation in Ireland - it was like Mexico," he tells Sky News.

"There were people being visibly kidnapped out of Dublin. There were two or three bank robberies in the country a day.

"You had armed men going into country towns and holding up three banks at the same time.

"It was chaotic."

Frustrated at the Gardai's failure to tackle Dublin's heroin problem through conventional methods, Michael began working with an undercover team known as the Mockeys who posed as drug users to catch dealers in the act.

But faced with the prospect of lengthy jail sentences, dealers would turn to violent tactics in a bid to escape arrest.

"Lots of us got fingers bitten," Michael says.

"You were getting bitten by guys who could be Aids carriers.

"There were a lot of injuries. A guy got hit with a hammer. One guy was bitten four times. (There were) black eyes, stitches.

"People lost teeth. One guy got a jaw fractured.

"The inner city was a tough place.

"A lot of these people were violent criminals anyway. You're stood between them and five years (in prison) - and they didn't care how they got away.

"They'd turn like animals. This was fight or flight.

"One guy on a top-storey balcony tried to push me over the balcony and I had to hang on for dear life... I was about five floors up.

"Looking back on it - it was hairy."

'Terrifying' undercover work

Michael says he never used drugs but had grown up in a tough part of Dublin - where someone in his class was "done for murder" - so he could "talk the talk" during his undercover work.

He also was "very slight", weighing 10-and-a-half stone, and at 5ft 9in tall, he only just met the minimum height requirement to serve in the Gardai at the time.

"You might sit on a wall or in a park with all these drug addicts for about an hour, an hour-and-a-half, swapping stories," he says. "Then you went and you did the buy.

"Was I frightened? I was terrified.

"You were operating on adrenalin.

"You don't have a radio. You leave your gun back at the office. You have your ID card in your sock.

"You go into these flat complexes and other drug addicts would mug you or rip you off.

"Some jobs didn't work out.

"You went in and just hoped for the best.

"It was terrifying but you're young, you feel invincible."

Michael spent about six years working undercover before going on to achieve the rank of assistant commissioner in the Gardai and then leading the EU's anti-drugs smuggling agency.

Now retired, he features in a new Sky documentary, Narcos Dublin, about the city's illegal drug trade, from the introduction of heroin in the late 1970s through to the 1990s as cocaine and ecstasy flooded into the country.

The three-part series, from the team behind the BAFTA-winning documentary Liverpool Narcos, charts how the notorious Dunne family rose to become one of Ireland's most terrifying gangs and looks at the murder of journalist Veronica Guerin, who had worked to expose drug barons.

Protecting family from 'darker side of life'

Michael, who arrested Micky "Dazzler" Dunne on drug charges, says it was "strange" to watch another member of the family, Christy, being interviewed for the series.

"It was like looking at something in the past to see him," he adds.

"It brought back memories - some of them not very good."

Michael says his family were unaware his work involved meeting dealers and pretending he wanted to buy drugs until they watched the documentary.

"My kids weren't around at the time - my wife knew I was off doing some sort of surveillance stuff and drugs stuff," he says.

"You see the darker side of life. When you come home, you don't talk about it.

"You close the door on it in your head. That's the only way... you don't worry the people at home."

Ex-addict who used heroin over 37 years

As well as featuring the efforts to tackle Ireland's illegal drug trade, the series hears the stories of former substance users including ex-heroin addict Paul Tracy.

He first injected the drug in Dublin at the age of 18 and continued using it over 37 years before finally going clean at the age of 55.

Now aged 59, the hairdresser says he was told by doctors he had just five years to live when he was 22 after testing HIV positive, which was linked to his heroin use.

"I had a promise of five years if I stayed healthy. If I was to use (heroin), I wouldn't last two years," he tells Sky News.

"I thought I would rather have two years on my terms.

"It was a self-destructive time."

He adds: "I was kind of excited. That was irrational.

"(I thought) 'Oh my god, I'm going to die young'. I had visions of my heroic, young death. Mad s**t. I can't even explain it to you.

"I couldn't wait to tell some people."

'Heroin takes your soul'

Despite his diagnosis in 1985, Paul says "incredibly" the HIV virus has now been undetectable for more than 25 years.

Describing his early heroin use, he says: "This thing made me feel really cool and relaxed and I liked the kind of person I was.

"Once the narcotic effects had worn off after an hour or two, I'd have this nice feeling - a false sense, maybe - that I was in control, and I was calm, and I was together.

"I actually liked this new person that came up in the middle of the drug. That was a very dangerous thing, that attraction to me."

But as his addiction developed - which at its height saw him taking two grammes of heroin a day - he turned to committing fraud to fund his habit.

"There's a poverty mentality around heroin because you never have enough," he says.

"Every time you see 20 quid, it's a get-well card.

"The obsession was so deep in me that I needed to break the obsession.

"I could go through the cold turkey all the time. I could never stay off it. The obsession was always with me. I needed something to break that.

"Everything else takes your money, your reputation. Heroin takes your soul.

"Nobody can take heroin and retain their soul."

Dublin Narcos is available to watch on Sky Documentaries and Now TV from today.