The court ruled unanimously that the rights of one cancer sufferer to an abortion had been breached because she did not have any "effective or accessible procedure" by which to access a lawful abortion.
The woman, identified only as "C", feared her cancer would relapse after she became unintentionally pregnant.
In largely Roman Catholic Ireland, abortion is banned except in very rare circumstances where the mother's life is deemed to be at considerable risk. Many women travel to Britain to have their abortion, where the laws are more relaxed.
The court also determined that "there was no explanation why the existing constitutional right had not been implemented to date".
The ruling means the Irish government may have to draw up new legislation authorising abortion beyond medical necessity.
Youth Defence described the ruling as "intrusive, unwelcome and an attempt to violate Ireland's pro-life laws".
Spokeswoman Rebecca Roughneen added that the court's finding was "not surprising" given previous rulings in support of abortion.
"The Court ruled that an 'existing Constitutional right to an abortion' had not been implemented to date in Ireland. But that's absolutely incorrect, since no such Constitutional right exists," said Ms Roughneen.
"This ruling should be dismissed out of hand by the government, since it is an unwarranted attempt to coerce the Irish people and overturn our ban on abortion.
"In fact, far from violating human rights, Ireland's pro-life ethos upholds and respects the human rights of both mother and child."
Niamh UÃ Bhriain of the Life Institute said the ruling was "agenda-driven" and "refuses to recognise the sovereign right of the Irish people to decide on important moral issues".
Professor William Binchy, head of the Pro-Life Campaign, said a referendum should be held before any changes to the law are made.
He warned that any move to introduce abortion on demand would compromise the mental and physical health of women.
He called for a "calm" and "respectful" national discussion on the issue of abortion.
"By all means, let us debate the abortion issue openly, honestly and with all the facts in front of us. But equally, we cannot shy away from the implications of what legal abortion would involve and the brutal reality of abortion, legal up to birth, in countries like Britain," he said.
"What's at stake in this debate is the value of life, and the sad experience is that once laws permitting abortion are introduced, they diminish the society's respect for the inherent value of every human life, born or unborn."