Ireland's prime minister, Simon Harris, gave a formal state apology on Tuesday to the families of 48 young people who were unlawfully killed in a fire at a Dublin nightclub in 1981.

The victims -- aged between 16 and 27 -- died in the early hours of February 14, 1981, when fire engulfed the Stardust ballroom in Artane, a north Dublin suburb.

On Thursday a 12-person inquest jury reached a verdict of unlawful killing in each of the 48 cases.

"Today we say, formally and without any equivocation, we're sorry," Harris said in an address to the national parliament in the Irish capital.

In the immediate aftermath of what was the biggest fire tragedy in Ireland's history, an inquiry found that the "probable" cause of the fire was arson -- a finding rejected by families of the victims as it appeared to blame those attending the disco.

That ruling was dismissed in 2009 and after years of campaigning by the families, the then attorney general granted a fresh inquest in 2019.

"We failed you when you needed us the most. From the very beginning, we should have stood with you but instead, we forced you to stand against us," said Harris.

"You were forced to endure a living nightmare when your loved ones were snatched from you in a devastating fire," he said.

"The state rubbed salt in your terrible wounds," he added.

The new inquests, which were the longest held in Ireland, began in April last year and heard evidence from 373 people.

The jury found that 42 people died in the nightclub premises and six people died following removal from the premises.

The cause of death for most cases was "rapid incapacitation due to inhalation of fire fumes and heat".

The fire started in a closet where hot water was stored in the main bar and was caused by an electrical fault, the jury agreed.

There was widespread shock in Ireland when the blaze ripped through the disco in about 20 minutes in the early hours of Valentine's Day, as the 800 attendees scrambled to escape.

The jury said that exit doors were locked, chained or otherwise obstructed, trapping many of the victims inside.

Half of the dead were aged 18 or under, while more than 200 people were injured.