Terminally-ill mother of two Marie Fleming has told the High Court she will die by taking gas through a facemask if the law allows her.

The 58-year-old who has multiple sclerosis, has lost the use of her limbs and is no longer able to take her own life without the help of her long-term partner Tom Curran.

She is challenging the constitutionality of the Criminal Law Suicide Act 1993 which can criminalise those who assist a suicide.

The former UCD lecturer claims the legislation violates her rights to privacy, dignity, and self determination and discriminates between her and an able-bodied person who could end their own life by their own hand.

Opening the case this morning her barrister Brian Murray SC said there is room for a narrow exception to the law on assisted suicide for people in her particular and extreme circumstances.

President Nicholas Kearns is presiding over the divisional court hearing the case.

'Looking for court assistance'

The 3 judges stepped down from the bench this afternoon and took seats in the body of the court room to hear Marie Fleming's evidence.

Confined to a wheelchair, she told them she was looking for the court's assistance for a peaceful dignified death.

She said she is losing her speech and the ability to swallow and that the pain she endures is sometimes unbearable.

If her partner Tom Curran assists her to take her life he could be prosecuted and jailed for up to 14 years.

The mother of two told the court that she wants to die in the arms of Tom and her children but she does not want to leave them that as her legacy.

She said she would use gas through a facemask rather than die a horrible death over months or a year.

Asked by her barrister Ronan Murphy SC for her thoughts on palliative care, she said it would not be acceptable to her to be kept comatose 'unable to smell the flowers or the changes in the season'.

The case continues tomorrow.