Gardaí investigating the disappearance of Jo Jo Dullard and Deirdre Jacob have recommenced a search operation on open ground in an area near the border between Wicklow and Kildare.
The Defence Forces assisted gardaí last week – under aid to civil power arrangements – in completing a ground survey of the area in Co Wicklow.
The land will now be searched and subject to excavation and forensic examinations over the coming days.
The search operation is being led by the Serious Crime Review Team, the Garda National Bureau of Criminal Investigation in conjunction with gardaí from the Kildare/Carlow Division.
Jo Jo Dullard was reported missing on 10 November 1995
It is being supported by the Garda National Technical Bureau and the Wicklow Garda Division.
A previous search of the area took place earlier this year, concluding on 25 February.
An Garda Síochána said the Dullard and Jacob families will be kept updated about the investigations and were informed of the search recommencing.
Gardaí are appealing to anyone with any information in relation to either investigation, no matter how small or insignificant to contact any garda station, or anyone who wishes to provide information confidentially can also contact the Garda Confidential Line on 1800 666 111.
Ms Dullard, who was 21, travelled to Dublin to socialise on 9 November 1995.
She missed the last bus home to Kilkenny and boarded a bus to Naas in Co Kildare instead, from where she intended to hitch a lift to her home in Callan.
Deirdre Jacob was last seen on 28 July 1998
After taking two lifts to reach Moone in Co Kildare, she phoned a friend at 11.37pm.
This was the last anyone heard from her. She was reported missing the following morning.
Deirdre Jacob was last seen at around 3pm on Tuesday 28 July 1998 crossing the road towards the entrance to her home at Roseberry, Newbridge, Co Kildare.
When last seen, the 18-year-old was wearing a navy v-neck T-shirt with white trim on the collar and sleeves, navy or black straight jeans and blue Nike runners.
She was carrying a distinctive black satchel-type bag with long shoulder-straps with CAT in large yellow letters printed on the front of the bag. The bag has never been found.