Focus Ireland has confirmed that the Unite union offered temporary use of its former headquarters to homelessness services some years ago.
Director of Advocacy Mike Allen said the offer was made to house the homeless.
A Focus Ireland spokesman said the property was reviewed by the charity and "found to be totally unsuitable for any of our projects or housing as it would have required extensive refurbishment, significant investment and planning permission".
He added that: "Given the offer was for three years or less it was not taken up."
Focus Ireland said the premises would only be available while the union was preparing its own plans for the future use of the premises.
Yesterday, RTÉ News reported that Unite had been granted an exemption from a social housing requirement when it applied to Dublin City Council for permission to re-develop the building.
This evening, Unite Education and Politics Officer Brendan Ogle said the union's members and the general public "can rest assured that Unite will continue to use all campaigning tools available on the issues that matter to working people and the communities they live in, including housing and homelessness".