An Irish man, whose sister and niece were killed in the 9/11 attacks, has told of his fear that the terrorists responsible for the atrocity could one day walk free.

Mark Clifford said he's endured "yet another massively frustrating year" in which no progress has been made in bringing to trial the five Guantanamo Bay-held suspects who were captured over a decade ago.

And speaking ahead of the 15th anniversary of the terror attacks, he said the ongoing failure of the US government to bring those responsible to justice has left his family and the loved-ones of the 3,000 victims of the atrocity in an "unbearable limbo".

Clifford's sister, Ruth McCourt and her four-year-old daughter, Juliana, were passengers aboard the second hijacked aircraft to crash into the twin towers of the World Trade Center on September 11, 2001.

In a cruel twist of fate, the Cork native had been told just hours before that his older, US-based brother, Ron, had narrowly survived the disaster when a ball of flaming wreckage fell on the building plaza in front of him.

However, Clifford said his still very raw grief has been compounded by the endless administrative hurdles and legal complications that have so far prevented a trial taking place in Guantanamo Bay.

And the 56-year-old said he is increasingly concerned that Khalid Sheikh Mohammed and the four other accused 9/11 plotters could evade justice altogether.

He said; "It's now 15 years since 9/11 and we haven't moved on at all in terms of bringing those responsible to justice.

"For the thousands of people like me, who lost family members and loved-ones, it's hugely frustrating. Just two years ago, David, Ruth's husband, died without seeing those who caused his wife's death brought to trial. And there must be hundreds more cases like that, where for example mothers and fathers of victims have passed away.

"There seems to be a lack of political will to get anything done, and that's extremely difficult for all the victims' family members, because we need to get closure.

"We're all extremely frustrated and I have a fear that some legal do-gooder could come in and get the five suspects off, which could let them walk free without facing trial."

Despite the lack of progress, Clifford said his older brother, Ron - who witnessed first-hand the devastation of the immediate aftermath of the terror attacks - is still expected to play a key role in the trial, if and when it gets underway.

But more immediately Clifford is preparing to privately celebrate the lives of his late sister and niece on Sunday, the day of the 15th anniversary of 9/11.

He added; "We'll be remembering Ruth and Juliana in our own way. We have suffered a massive loss and I still struggle to come to terms with what happened, because it was so horrific.

"Juliana should be a happy, 19-year-old young woman now, maybe entering her second year of college or travelling the world. So many lives were destroyed that day, and the anniversaries don't get any easier."

Read more: 9/11 - fifteen years on