The sister of missing man Trevor Deely says someone, somewhere knows what happened to her brother.
Michele Deely was speaking as Wednesday marks National Missing Persons Day.
Trevor attended a work Christmas party in Dublin city centre on December 7th, 2000 and has not been seen since.
Trevor had called to his place of work to collect an umbrella for his journey home in the early hours of December 8th.
CCTV footage captured a man dressed in black outside the rear entrance of the Bank of Ireland Asset Management Offices on Wilton Terrace, between 3.00am and 3.34am, before Trevor arrived.
At 3.34am this man steps onto the footpath and follows Trevor to the back gate of the offices, before talking with him at 3.35am.
Trevor was last seen on CCTV passing a Bank of Ireland ATM machine on Haddington Road at approximately 4.14am.
Gardaí believe the man acting suspiciously outside of Trevor's workplace is the same man that is seen passing the Bank of Ireland ATM on Haddington Road 34 seconds after Trevor.
Trevor's sister Michele told The Pat Kenny Show time does not heal everything.
"It doesn't feel like time does heal, actually.
"Every year I say certainly - and I know other members of my family - that we just can't go through this again for another year.
"And yet we've no choice, you just have to; so I think it actually gets harder. It just gets harder every year that you're still stuck in the same place.
"21 years is a lifetime - it's such a long time - so much has transpired in the world, so much has transpired in our lives.
"Yet we are stuck in this moment in time, but what's not stuck is my parents are a year older again.
"It's 21 years of trauma so our appeal remains constant in asking people to help us come forward and find Trevor."
'We've tried everything'
Trevor, who was 22 when he went missing, would now be 43.
Michelle says they have received incredible support, but nothing has come to fruition.
"We've tried everything - in all jurisdictions as well, not even just in Ireland - we have tried every single thing that we possibly can.
"And yet still absolutely nothing, and that's kind of soul-destroying.
"My parents talk about 'keeping hope', but it's quite hopeless to think 'What else do we need to do?'
"And it's just that constant appeal to the person or persons who do know something - and we know that.
"We all know that somebody knows: that man... has not been identified, that person spoke with Trevor - that's real, it's captured on CCTV.
"My parents, as I said, are another year older - it's just, if for no other reason, to give them some peace to just please come forward".