An Taisce previously warned the museum has "no spare space for the ‘temporary’ convenience of vagrant Senators"
Seanad Éireann will temporarily relocate to a room in the neighbouring National Museum while the Seanad chamber is refurbished.
It comes despite objections being raised over the move to the Ceramics Room of the museum, including from An Taisce.
As part of the deal, the Houses of the Oireachtas will fund the refurbishment of an area for future use by the museum as an exhibition space. Funds will also be made available for the construction of a new lift.
In a statement, Ceann Comhairle Seán Ó Fearghaíl said: "We are committed to ensuring that the integrity of the museum will be maintained and respected at all times during the relocation, which is expected to run for a period of 18 months.”
"As has been noted before, the Georgian part of Leinster House requires urgent and extensive restoration work to preserve the building for future generations, and we very much appreciate the cooperation of the National Museum in relation to this important conservation project.”
In a complaint published earlier this month - their second complaint over the plans - An Taisce said that a pit dug in preparation for the proposed lift "was dug in the absence of either planning permission or an exemption".
The museum building is protected, and the national trust argues that it has "no spare space for the ‘temporary’ convenience of vagrant Senators" due to existing plans for structural alterations.
Dr Mark Clinton of An Taisce’s Monuments & Antiquities Committee said: “Why should an accommodation issue in Leinster House become a problem for the National Museum? What have the Senators got to say for themselves? Their silence is deafening.”