The Supreme Court has ruled that the Pro Life Campaign should not be allowed to participate in the forthcoming appeal in relation to the rights of the unborn.
Lawyers for the group told the court that arising from "concerns over recent developments", it wished to be joined to the action as an amicus curiae - an assistant to the court on legal issues.
They told the court that the group's expertise and experience extending over 25 years in protecting the rights of women and their unborn children would be of assistance to the court in deciding the legal issues.
The application was opposed by the State, who argued that the case concerns Irish constitutional law and the Pro Life Campaign does not meet the necessary legal test to join the action, as it cannot bring any expertise the lawyers already involved do not have.
In the court's ruling, Mr Justice Donal O'Donnell said the court was not satisfied that the issues that the court should properly address in this appeal could not properly, adequately and perhaps best be advanced by the existing parties in the proceedings.
The appeal is due to get under way on 21 February.
The State has appealed against a finding by the High Court in 2016, that the "unborn" has constitutional rights in addition to the right to life in the Eighth Amendment.
Mr Justice Richard Humphreys in the High Court ruled that the unborn was a child within the meaning of Article 42A of the Constitution and has rights the State is required to protect and vindicate.
The State will argue the High Court got it wrong in deciding the unborn has constitutional rights extending beyond the right to life.
The appeal arises from an immigration case involving a Nigerian man, his Irish partner and their child, who was not yet born when the legal proceedings were initiated.
The court heard there is no dispute between the sides on the facts of the case.
The court's judgment may affect the wording of the forthcoming referendum on the repeal of the Eighth Amendment, which gives the unborn an equal right to life to that of its mother.
The court and the State have previously said they are anxious to have the case heard and decided as quickly as possible in light of the referendum.
In his ruling, Mr Justice Donal O'Donnell noted that it was the function and duty of courts to uphold the Constitution. He said courts must interpret the Constitution, but he stressed that a court was not free to make observations in general on social, political or even legal issues.
He said a central feature of the court's function was to decide the law only within the specific context of the litigation before it and for the purposes of deciding that case.
Mr Justice O'Donnell also noted that these were immigration proceedings. The Nigerian man who was facing deportation had argued in the High Court that the Minister for Justice was obliged to take into account the fact that his partner was pregnant.
Lawyers for the minister argued that she did not have to consider the pregnancy or the rights or interests of the child not yet born.
The judge said when the High Court judgment was delivered in July 2016, it was immediately recognised as having potentially wide-ranging implications.
But he said no application to join the proceedings was made in the immediate aftermath of that judgment.
He said it was an almost unavoidable inference that the application by the Pro Life Campaign was precipitated by the increased discussion of the case in the context of the forthcoming proposed referendum.
Mr Justice O'Donnell also said that if the people of Ireland are to be asked to change their basic law, it was at a minimum desirable that if possible there should be no avoidable uncertainty about what the law is.
He said it was accepted that permitting a person to intervene in proceedings as an "amicus curiae", or a friend to the court, was a matter for the discretion of the court.
He said the fundamental question to be addressed was whether the court was likely to be assisted significantly by the intervention offered.
Mr Justice O'Donnell said what was before the court was a pure issue of law. No medical or social issue had to be determined and the only expertise in issue was legal.
He said the respondents in the case were represented by an experienced legal team who were involved in the High Court hearing.
They had rejected any suggestion that they were at a disadvantage because of the way the case had been advanced and the judge pointed out that the State had agreed to be responsible for the respondents' costs.
He also found that the Pro Life Campaign had not identified any particular legal argument which it wanted to make or thought the campaign could make to particular effect.
Mr Justice O'Donnell said no measurable concern had been established to the satisfaction of the court which would suggest that the full range of argument would not be made skilfully and forcefully on the legal issues that arose in the case.
He said it seemed the assistance that could be offered by the Pro Life Campaign at this stage was minimal.
Mr Justice O'Donnell said the application was clearly being made against the background of an incipient referendum campaign.
He said the constitutional position of the unborn was not the only issue in the appeal. And he said the Pro Life Campaign was not the only group concerned with that issue.
He said if the campaign, who are admittedly partisan, were allowed to participate in this appeal, it would be significantly more difficult to reject applications from other groups.
He said this would risk the case being delayed or postponed. And he said there would be a risk that more general arguments would be advanced which would exert a "gravitational pull" away from the issues in the case.
He said the court was not satisfied the issues the court should address in the appeal, could not be properly, adequately and perhaps best advanced by the existing parties in the proceedings.
Outside court, Caroline Simons of the Pro Life Campaign, said they were disappointed with the decision.
But she said they had been assured by the court that the issues would be given the "widest possible articulation" with the "greatest possible legal expertise".