The pro-amendment psychiatrist, Prof Patricia Casey, has conceded that divisions in her profession over the referendum could pose difficulties for suicidal pregnant women seeking mental health care.
Prof Casey said there was no room for compromise after 10 of her colleagues expressed concern at the stance she and fellow psychiatrist Prof Anthony Clare shared on the referendum.
She said to allow terminations in even the rare cases which the 10 psychiatrists highlighted would lead the State on the "slippery slope" to liberal abortion.
She did not understand why the 10, who said in a letter to The Irish Times that ruling out suicide as grounds for abortion would endanger some women's lives, had not spoken out publicly before. The debate had been running for 10 years.
Prof Casey was speaking after an appearance at University College Dublin, where she was guest of the student group UCD Yes To Life Campaign. She said all psychiatrists would "act professionally" in their care of suicidal pregnant patients but repeated her belief, outlined in a joint statement with Prof Clare, that there were no psychiatric grounds for abortion.
"We said in our joint statement that psychiatrists would become hopelessly compromised. Indeed some people would choose the psychiatrist they went to as a result. There is that danger. That is the difficulty," she said.
Earlier, Prof Casey had answered questions from an audience of more than 300 students from both Yes and No campaigns.
She said the signatories to The Irish Times letter appeared to rely heavily on the stance adopted by the Royal College of Psychiatrists in London, which allowed abortion on mental health grounds up to the 24th week of pregnancy and later in certain cases.
She said it would not be possible to allow abortion here without "importing" British law as the boundaries of restrictive abortion would be continuously pushed out. "I am confident that within five years there would be a demand from people saying suicide as a risk is too restrictive - we need to broaden it out.
"Within 10 to 12 years, we would have abortion up to 24 weeks as the norm and up to birth in certain circumstances, she said. "I agree that it would open the floodgates. I do believe in the slippery slope theory in relation to abortion."
Prof Casey said it was not possible to compromise. "Do we want to legalise abortion or do we not? It's as simple as that."