The State had been advised that a legal mechanism used to inquire into the death of 14-year-old Brian Rossiter after his detention at a Garda station contained "serious drawbacks" and was likely to work "against getting at the truth", according to previously undisclosed legal advice.

The newly discovered legal assessment is contained in a briefing note on the so-called 'Kerry babies case' from December 1984, at which time the option of using the Dublin Police Act 1924 was dismissed as inadequate by then-Justice Minister Michael Noonan.

Minister Noonan ultimately opted for a sworn inquiry in the Kerry case, but the Dublin Police Act 1924 was later controversially used by his successor, Michael McDowell, when he established an inquiry into the circumstances surrounding the death of the teenager in September 2002.

Speaking on RTE's This Week programme, the solicitor for Brian Rossiter's family, Cian O'Carroll, said that if the legal advice from the mid-1980s had been known when the Rossiter inquiry was set up in 2005, it would have "quite likely" led to a different type of inquiry.

He said the concerns raised in the mid-1980s were similar to those raised by the family in 2005.

Brian Rossiter was arrested on the evening of 10 September, 2002, at Upper Gladstone Street, Clonmel, and detained at Clonmel Garda station until the following day.

He lost consciousness during his time in detention and, following resuscitation, he was taken to St Joseph's Hospital, Clonmel. He was later transferred to Cork University Hospital where he died on the afternoon of 13 September, 2002.

The Rossiter family alleged he had been assaulted in custody, which the state denied. Following a civil lawsuit, the State ultimately agreed a settlement with the Rossiters of €200,000 in relation to their child's death, but without any admission of liability on the state's part.

At the time the inquiry into his death was launched in 2005, the Rossiter family strongly opposed the use of the largely obscure 1924 Act rather than a sworn enquiry.

The family's solicitor, Cian O'Carroll, said they had resisted its use on the basis that they believed the 1924 Act was unsuited to establishing the facts behind why the teenager died, and was instead limited to a procedural assessment of whether specific serving gardaí had broken the law.

At the time he launched the inquiry, the then-Justice Minister Michael McDowell defended the use of the 1924 Act.

But according to the newly released state papers, the State had received legal advice as far back as December 1984 saying there were problems with this legislation.

In a briefing note prepared for Cabinet, then Minister Michael Noonan concluded in December 1984, that there were a number of "serious drawbacks" which, in the minister's view, precludes its use.

First, it said, the wording of the 1924 Act would mean the inquiry would have to be confined to alleged neglect or violation of duty by members of the force - it would necessarily narrow the scope of the inquiry and restrict it from going into all aspects of the case.

Second, there is no provision of privilege for witnesses to protect them against defamation; and statements made to the inquiry could be used in subsequent criminal proceedings unlike other a sworn inquiry.

He concluded: "These are features which, apart from causing legal difficulty, would militate against getting at the truth of what occurred".

The then minister for justice Michael McDowell responded to the initial criticisms of the use of the 1924 Act at the time the terms of reference were outlined, in July 2005.

At that time his spokesperson issued a statement saying that the "the terms of reference will enable Mr Hartnett to inquire into all the circumstances surrounding the arrest and treatment in custody of Brian Rossiter and the role of all gardaí who came into contact with him over the relevant period," the department said.

"The Minister [Michael McDowell] is completely satisfied that this can be achieved through the inquiry format which has been decided upon, i.e. the Dublin Police Act 1924, as amended," the statement added.