An Irish man, whose sister and niece were killed in the 9/11 attacks, has admitted his hopes of seeing those responsible for the atrocity brought to trial have faded under Donald Trump's presidency.

U.S. prosecutors are seeking a 2019 trial date for five Guantanamo Bay-held prisoners accused of masterminding the terrorist attack, which took place 16 years ago today.

But Mark Clifford – who will today join other members of his family in his native Cork to privately celebrate the lives of his sister Ruth McCourt and her four-year-old daughter, Juliana – said he believes it's highly unlikely that the long-awaited trial will take place whilst Trump is president.

As things stand, Trump will remain president until at least January 20, 2021, barring an impeachment, resignation or assassination – and up to 2025 if he successfully runs for re-election.

Read more: Family of 9/11 victims slam delay in justice process

That means the families of the 3,000 victims could be facing an even lengthier wait for justice than previously anticipated.

Clifford, 57, said: "The past year has been very frustrating – just like all the years since 9/11. No progress has been made in terms of bringing those responsible to justice. And, to be honest, I don't hold out much hope of that changing under Trump's presidency.

"It just doesn't seem to be on Trump's radar and it's not something you really see him tweeting about. I just don't think getting the trial underway is a priority for the US administration. There doesn't seem to be the political will there to do anything about it."

Clifford said the ongoing impasse over 9/11 has made it impossible for the families of the nearly 3,000 victims of the atrocity to move on.

He added: "It doesn't get any easier, because we remain in a state of limbo. The grief doesn't just disappear, particularly when we haven't yet got closure. And the longer this goes on, the more there'll be family members who unfortunately pass on without seeing justice for their loved-ones. We've unfortunately been in this situation, having lost a number of family members, since 9/11."

Read more: Remembering from afar – 9/11 memorials in Ireland

Along with losing his sister and little niece – who were passengers on the second hijacked aircraft to crash into the Twin Towers – Mark's older brother Ron narrowly survived the disaster when a ball of flaming wreckage fell in front of him on the building plaza of the World Trade Center.

Ron Clifford

Ron Clifford

When the long-awaited trial does eventually go ahead, the five accused 9/11 plotters will face a military commission at the U.S. base in Cuba on charges that include terrorism and nearly 3,000 counts of murder in violation of the law of war.

They could get the death penalty if convicted by a jury of military officers.

However, to date, the start of the trial has been hindered by endless administrative hurdles and legal complications. The case has also been bogged down by pretrial motions, as lawyers for the accused seek classified evidence about their treatment by the CIA.

Read More: Ron Clifford escaped World Trade Center, as sister and niece killed in plane

Juliana and Ruth McCourt