An Garda Síochána upgraded her case from a missing persons case to a murder investigation soon after the young woman was discovered to be missing.
Continuing to liaise closely with the Collins’ family, gardaí are renewing their appeal for information and are asking the public if they have any piece of information which they feel relevant to this investigation to make contact with them.
Sandra would have turned 50 this month and her family and An Garda Síochána are encouraging anyone who might have information which perhaps they thought was insignificant at the time to come forward.
The last confirmed sighting of Sandra Collins was on the night of Monday December 4, 2000 at approximately 11pm at the Country Kitchen premises George’s Street, Killala County Mayo.
The 28-year-old found out she was pregnant on the morning of her disappearance. She left her aunt's home where she lived and visited shops in the nearby village before heading to a takeaway where she purchased a bag of chips.
The fleece jacket she was wearing that evening was found at Killala Pier days later.
Speaking to mark National Missing Persons Day, her younger brother Patrick described Sandra as “the big sister that everybody would have wanted” and said that she was a “fun, kind, vivacious” person.
He said that after 21 years, his family just wanted to find Sandra’s body and give her a proper burial.
He said: “[Finding her remains] would be like winning the €19 million [jackpot], but it would be 100 times better.
“I'd give every single penny of that. If I won it today... I'd hand every penny over to whoever could give her back to me. Whether it was her killer or not... I don't care.
“She was a lovely person and she doesn't deserve to be dumped in a bog, ditch or drain - she deserves a Christian burial.
“You can’t let it consume your life because there’s no point in that and she wouldn’t want that either. But at the same time, it’s still a part of you. There isn’t a day goes by that we wouldn’t mention her or talk about her.
“We try and live our lives... we’ve done our own things, my sisters had children. She has nieces and nephews that will never know their aunty Sandra.
“We live our lives but Sandra’s [disappearance] is there every single day of the week the last 20 years.”
“It’s especially hard this time of year,” he added.
In 2014, a man local to the area where Sandra lived, Martin Earley was put on trial for her murder but was ultimately found not guilty.
The plasterer however admitted to the court that he had a sexual relationship with her around the time of her disappearance, but denied being involved in her death.
In a statement, Gardaí said any person who comes forward with information will be treated with confidentiality, compassion and sensitivity.
Anyone with information can contact Ballina Garda Station on 096-20560, any Garda Station, the Garda Confidential Line 1800 666 111 or Crimestoppers 1800 250 025.