The case could force Ireland to amend its near total ban on abortion, the BBC reports.
While abortion in the Republic is technically allowed if a woman's life is at risk, the court said that was not made possible for the woman involved.
The court said the Irish government had failed to properly implement the constitutional right to abortion if a woman's life was in danger.
Irish Prime Minister Brian Cowen said politicians now needed to consider the implications of the ruling.
The woman feared the pregnancy could trigger a relapse of her cancer, which was in remission at the time.
The court ordered Ireland to pay 15,000 euros in damages to the woman, who was forced to travel to Britain to terminate her pregnancy.