The High Court has ruled that the first meeting of the Seanad can only lawfully take place with its full membership of 60.
The three-judge court gave its decision in the action taken by ten senators, aimed at clarifying whether or not the Seanad can sit and pass legislation before the Taoiseach's 11 nominees are appointed.
The senators wanted a declaration from the court that there is no constitutional requirement for all 60 members of the Seanad to be appointed before it can sit and pass legislation.
The State argued that the Seanad was not properly composed until all 60 members were appointed.
The senators had sought an urgent judgment as some legislation, including legislation governing the operation of the Special Criminal Court, needs to be renewed today or it will lapse.
However, the Seanad is meeting today at the Convention Centre in Dublin, after Taoiseach Mícheál Martin announced his 11 nominees on Saturday.
This means the importance of the judgment will be to provide guidance if a similar situation arises in the future.
In the ruling, the court said it was satisfied that the Constitutional provision in relation to the membership of the Seanad, clearly and unambiguously meant that the Seanad must be comprised of 60 members, 11 of whom are nominated, and 49 of whom are elected.
It rejected the argument that the Seanad was entitled to sit before the Taoiseach had nominated the 11 members provided for in the Constitution.
It also said the senators who took the case were wrong to describe what had occurred since 29 March, when the members of the outgoing Seanad, ceased to hold office as a "constitutional lacuna".
The judges said it was simply the product of the form of democracy adopted in the Constitution.
They said this proceeded on the basis that the organs of State to whom powers are conferred by the Constitution, would act to give effect to them and elected representatives would act rationally and responsibly to ensure a government was put in place.
The court said it understood the genuine concern of the ten senators that without a functioning Seanad it would be impossible to enact emergency laws, but it did not believe such considerations could be allowed to influence the interpretation of the Constitution.
The ten plaintiffs are elected senators Ivana Bacik, Victor Boyhan, Gerard Craughwell, Annie Hoey, Sharon Keogan, Michael McDowell, Rebecca Moynihan, Rónán Mullen, Marie Sherlock and Mark Wall.