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Pleading guilty to a drink driving charge at Limerick District Court in Ireland recently, Fiona Porter, 24, put an unusual plea in mitigation. She explained through her lawyer that she had drunk alcohol on the evening in question in a distressed state after finding her husband in bed with her mother.

The court was told that after catching her husband Hugo, 34, and her mother Bernadette Garvey, 53, in flagrante delicto she learnt that they had been having an affair for two years.

The drink driving case was very serious, as Mrs Porter was already banned at the time of the offence, but the judge acceded to her lawyer’s request for leniency and spared her a custodial sentence. He suspended a four-month prison term and banned Mrs Porter from driving for six years.

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Then, however, the drama took another twist. "The way I feel about her this minute, I would stick a f***ing knife in her,” Bernadette Garvey said of her daughter. Ms Garvey denied the affair and declared, “I hate her and I wish to God she was dead for doing this to me."

After the court hearing, investigations revealed that on June 28, the night Hugo Porter was alleged to have been in bed with his mother-in-law, he was in fact tucked up elsewhere, embraced in a most secure alibi: the B-wing of Limerick Prison. He had been there since May 31, doing six months for a series of serious motoring offences.

But the case didn’t end with that twist. Mrs Porter has now alleged that her lawyer is a liar. She said she had actually caught her mother and husband in bed on May 22, not on the June evening when she was stopped for drink driving, and that her lawyer knew that. Her lawyer denies that he misrepresented his client and says he followed instructions precisely.

It remains to be seen whether the court will want the matter brought back for further consideration.

At the trial, neither Mrs Porter nor her husband gave testimony. Mrs Porter was exercising her right to silence and Mr Porter was exercising in the local jail. It is difficult to know the truth at the heart of the dispute but I hope that the thoughts of a witness in an old English case don’t apply. The distinguished Irish barrister Maurice Healy recounted that an English judge, Mr Justice Darling, once got very impatient with a witness from Ireland whom he suspected of not being fully frank.

“Tell me,” the judge snapped, “in your country, what happens to a witness who does not tell the truth?”

“Ah, me Lord,” the witness replied candidly, “I think his side usually wins.”

Professor Gary Slapper is Director of the Centre for Law at The Open University

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