The mother of Annie McCarrick has said she would "love to be able to find" her daughter and her wish is to bring her home.
Speaking to RTÉ News from her home in Bayport, Long Island, New York, Mrs McCarrick said the move by gardaí "means they are going to start from the very beginning".
She said she hopes the decision will mean gardaí will now review all the evidence and information received to date.
"That's what they said they will do, which is just great, really it is," she added.
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"I think it's the best we could hope for."
Asked if the McCarrick family was holding out any hope that Annie could still be alive, Mrs McCarrick said "no, I didn't think that was remotely possible".
"I did for a very, very, very long time but not after 30 years," she said.
"No, no, I don't think that would be possible."
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She said the most important thing now is to be able to bring the remains of Annie home to Long Island.
"I mean, whatever did befall her, it may have been accidental," Mrs McCarrick said.
"I would just love to be able to find her."
She said Annie "certainly loved it in Ireland" but Mrs McCarrick said it would be "my wish" to be able to bring her home again.
"She absolutely loved it," she said.
"She loved everything about it. She really did."
Mrs McCarrick said it would be great if the decision to reclassify the investigation could finally provide some answers to the family.
She appealed to anyone with any information about the disappearance of her only child to speak to the gardaí.
"There is nothing more important, I don't think really, that anyone could do then if they had any - even just the smallest bit of information - you never know how helpful it might be," she said.
Mrs McCarrick said she wonders "if someone perhaps may have seen her someplace else but doubted it, you know because they thought well, that was in a different place".
"If they have any, any, any, the smallest bit of information, if they would kindly let the gardaí know, it would be greatly, greatly appreciated," she said.
Asked how difficult the last 30 years have been for her family, Mrs McCarrick said "well time helps a great deal, it really does".
"You do the best you can and you just go along, you don't have a choice," she said.
She said "it's just been a very long time and I'm very, very grateful that the gardaí are going to upgrade her case, very grateful".
Annie, who lived in St Cathryn's Court, Sandymount in Dublin, was aged 26 when she went missing on 26 March 1993.
The last confirmed sighting of Ms McCarrick was captured on CCTV shortly before 11am that day at the AIB bank on Sandymount Road close to where she lived.
There are also reported sightings of her in the Sandymount Green area, reported sightings of her boarding the number 44 bus bound for Enniskerry in Co Wicklow and a number of further reported sightings in Enniskerry village and at Johnny Fox's pub in the Dublin mountains.
At a press briefing this morning, Detective Superintendent Eddie Carroll said gardaí are "satisfied that it is more likely now that Annie came to a foul demise".
Annie was an only child and her father John passed away in 2009.
Ms McCarrick is described as 5'8" in height, 10 stone and long brown hair.
Gardaí said she spoke with a soft Irish-American accent and when she disappeared it is believed she was in possession of a large brown leather bag.
Speaking today, Det Superintendent Carroll said: "I would urge any person or persons with information in relation to the Murder of Annie McCarrick to please come forward to either the investigation team at Irishtown Garda Station at 01 6669600, your local Garda Station or the Garda Confidential telephone line 1800 666 111.
"I appeal to any person who have information relative to Annie's murder not to assume we know and/or that it has limited value. Let us make that decision."
