The 51-year-old, of Barnhill, Castledermot, was found guilty late last year of murdering 31-year-old Ms Cummins and of assaulting Mr Lawlor (44) on May 13, 2010.
He had pleaded guilty to the manslaughter of the mother-of-one but not guilty to her murder at Michael Dooley Terrace, Athy. McDonald had denied assault causing harm to Mr Lawlor, of Pearse Terrace, Castledermot on the same occasion. However, a Central Criminal Court jury found him guilty of both crimes.
Detective Sergeant Sean Boland told the court yesterday that six weeks before the offences McDonald and Ms Cummins, who were both alcoholics, had ended their five-year relationship.
She moved out and began a relationship with their acquaintance, Mr Lawlor, moving into the house on Dooley Terrace with him.
Det Sgt Boland explained that around 11pm on the night of May 12, McDonald armed himself with a steak knife and took a taxi the 18km from his home to Dooley Terrace. He had been drinking and taking tablets, but not beyond his usual level of consumption.
He knocked on the door, which was answered by another tenant. He brushed past this man and went upstairs to the couple's bedroom.
He announced his intention to take Breda Cummins away with him, but Mr Lawlor said no. He produced the knife and made a stabbing motion at Mr Lawlor, prompting him to raise his hand defensively. In doing so, he received a penetrating injury to his right hand.
The detective agreed with the prosecution that Mr Lawlor then ran downstairs, leaving the accused alone with Ms Cummins. McDonald stabbed her six times in the heart and lungs as she lay on the bed.
Ms Justice Iseult O'Malley said that it had been a "sad and tragic trial".
She said the long-term impact on Mr Lawlor was more likely to be the psychological effects of Breda Cummins's loss of life. She imposed a three-year sentence to run concurrently with the life sentence McDonald is serving for murder.