Speaking outside court after McDonald was sentenced to life for the murder of Breda (31), Sabrina Cummins said her family were "glad we got justice".
"I didn't like Michael when I met him first. I knew he wasn't good for my sister. He was beating her a few times and maybe once or twice he would turn around and say to me that I was gorgeous and not to tell Breda," she said on the steps of the courthouse.
Breda's other sister, June, said their family remain devastated and shocked by Breda's death, especially their mother Barbara who is "very angry, upset and crying all the time".
"Breda didn't deserve what happened to her," June said.
"She wasn't perfect and had problems, but she didn't deserve what happened to her. No one deserves a death like that."
McDonald stabbed Breda six times in the chest in what was described as a "shocking, sad and outrageous death" by the trial judge. He was handed a mandatory life sentence after he was found guilty by a jury at the Central Criminal Court.
McDonald was also found guilty of assault causing harm to John Lawlor (44) -- Breda's new partner.
The jury returned a unanimous verdict of guilty on both counts after two hours and 41 minutes of deliberations. McDonald showed no reaction to the verdicts.
McDonald (51), of Barnhill, Castledermot, Co Kildare had pleaded not guilty to the murder of Breda Cummins on May 13, 2010 at Michael Dooley Terrace, Athy.
He had pleaded guilty on arraignment to the manslaughter of the mother-of-one, but this plea was not accepted by the DPP.
McDonald had been in a relationship with Ms Cummins, who was also an alcoholic.
He had told gardai it was a "tempestuous relationship", and that Ms Cummins started going out with Mr Lawlor several days after the break-up.
The 13-day trial heard that McDonald, who had been drinking on the evening of the murder and had taken sleeping tablets, brought a kitchen knife from his home in Castledermot to Michael Dooley Terrace to "rescue Breda".
McDonald arrived at the house at around midnight and asked to see Ms Cummins. Mr Lawlor and the house owner, Michael Fennell, were downstairs when Ms Cummins told McDonald to come upstairs. Mr Lawlor followed him to the bedroom.
A fight broke out between Mr Lawlor and McDonald and he was stabbed in the hand by McDonald with the kitchen knife.
After Mr Lawlor left the room to tend to his hand, McDonald turned to Ms Cummins, stabbed her six times in the chest with the kitchen knife and said, "I love you."