The sister of a teenager who was killed in the Omagh bombing believes the conclusion of the inquiry will bring out “a very diluted version of the truth”.

Claire Hayes, whose brother Alan Radford was killed in the bomb when he was 16, said she was sceptical of the outcome of the inquiry due to what she believes is a lack of transparency from government bodies.

The attack, carried out in 1998 by the Real IRA (RIRA), was the biggest single atrocity of the Troubles in Northern Ireland, killing 29 people, including a woman pregnant with twins.

Ms Hayes is addressing the inquiry on Wednesday.

Speaking to BBC’s Good Morning Ulster on Wednesday, Ms Hayes described her brother Alan as “the most innocent, purest, kindest human”.

“It pains me so much to know he died alone on the street in such a barbaric way,” she said.

Ms Hayes believes the families will get a conclusion as to whether the bomb could have been stopped but doubted that the full truth will emerge.

“There are too many redactions that can be made, there are too many protections in place for the likes of MI5 or informants or whoever,” she continued.

“When the Irish government isn’t partaking in it, it’s not a full inquiry so for me there’s already a flaw.

“We need complete transparency from the southern government.”

She added that while the Irish government has said it will help “they’re not compelled to do so and, for me, for Alan, that’s not something I wanted”.

“I wanted it to be completely robust and completely transparent for him so we know absolutely everything.”

Taoiseach (Irish prime minister) Micheál Martin said on Tuesday that he has made it “very clear” that the Irish government must cooperate fully with the inquiry.

Tánaiste (deputy Irish prime minister) Simon Harris said the Irish government will “not be found wanting”.

He said that the “cross-border nature of the assistance, to an inquiry established under UK law, means that it takes some work to get the details of the mechanisms of cooperation right, but we are actively engaging in this work”.

On Tuesday, the inquiry opened with its chairman Lord Turnbull stating he was “shocked at the level of grief” imposed on bereaved families and survivors.

The names of those murdered in the attack were read out and a minute’s silence observed at the commencement session.

Day one of the inquiry heard tributes to two Spanish victims of the bombing.

Paul Greaney KC, senior counsel to the inquiry, said the statements and personal testimony were a key starting point in the inquiry.

Earlier, Michael Gallagher, who lost his 21-year-old son Aiden in the attack, said he was “grateful that the victims are centre and front and a voice will be given to them”.

“I think it’s important the world knows the type of people we lost in Omagh that day,” he said.

The inquiry seeks to determine whether the bombing could have been prevented by UK state authorities.

This phase of the inquiry, at Strule Arts Centre in Omagh, will last four weeks and is also hearing evidence from survivors and first responders.

The Irish government has promised to co-operate with the inquiry – the bombers launched the attack from across the border.

Dublin previously indicated there was no new evidence to merit a separate public inquiry in the Republic of Ireland.

The bomb exploded in Omagh town centre on a busy Saturday afternoon on 15 August 1998.

The attack took place four months after the signing of the 1998 Good Friday Agreement.

The RIRA – a dissident republican paramilitary group – disagreed with the Provisional IRA’s decision to call a ceasefire ahead of the talks and continued to plant car bombs in towns across Northern Ireland.

But the Omagh bomb was by far the RIRA’s most deadly attack.

Nine children, including a baby, were among the dead.

More than 200 other people were wounded, some of whom survived with life-changing injuries.

Three days after the 1998 attack, the Real IRA released a statement claiming responsibility for the explosion.

It apologised to “civilian” victims and said its targets had been commercial.

Almost 27 years on, no-one has been convicted of carrying out the murders by a criminal court.

In 2009, the judge in that case ruled four of the men – Michael McKevitt, Liam Campbell, Colm Murphy and Seamus Daly were all liable for the Omagh bomb.

The four men were ordered to pay a total of £1.6m in damages to the relatives, but appeals against the ruling delayed the compensation process.

A fifth man, Seamus McKenna, was acquitted in the civil action and later died in a roofing accident in 2013.