The High Court has found the first meeting of the Seanad can only lawfully be held with 60 members in place.
10 senators took a case, seeking a declaration the upper house can meet and pass legislation without the taoiseach's 11 nominees.
In a 62-page judgement, the three judges of the High Court hearing the case rejected arguments from the 10 senators that the Seanad can legislate with 49 members.
The senators - including Ivana Bacik, Ronan Mullen, and Michael McDowell -were disputing the State's view that the upper house can only sit when the taoiseach nominates their appointees.
In today's judgement, the court said it was satisfied Article 18.1 “clearly and unambiguously" means the Seanad “must be comprised of sixty members, eleven of whom are nominated and forty nine of whom are elected”.
The judges said: "The Court acknowledges as genuine and sincere the plaintiffs’ concerns that without a functioning Seanad it would be impossible to enact emergency legislation that might at any time be required and that the arrival of the COVID-19 pandemic has served to highlight the significance of the risk to which the State might be exposed.
"While we understand why the plaintiffs have raised this issue, we do not believe that considerations of that kind can be allowed to influence the interpretation of the Constitution."
The case had some urgency as legislation around the Offences Against the State Act and the Special Criminal Court would lapse at midnight tonight if a government wasn't formed.
A new Government was formed over the weekend, with the Taoiseach Micheál Martin's 11 nominees confirmed on Saturday evening.
The full Seanad is due to meet today for the first time at the Convention Centre Dublin.
