Slain Massachusetts mom Ana Walshe spent Thanksgiving in Dublin with a mystery lover about a month before her husband killed her on New Year’s Day, new court filings alleged.

The unidentified beau also told police that he’d been having an affair with the mother-of-three for several months, according to the Daily Mail.

Walshe, 39, a real estate executive from Cohasset, Massachusetts, mysteriously disappeared Jan. 1 and has been presumed dead, even though her body has not been found.

Authorities have charged Brian, her 47-year-old husband, with first-degree murder. Police say they found damning evidence that links him to the crime, such as her clothing and Google searches about how to dismember and dispose of a body allegedly made on his son’s iPad.

Prosecutors said he beat his wife to death after discovering her affair, then chopped her up in the family basement. He was arraigned Thursday in Norfolk Superior Court.

Days before her death, Ana told a friend she thought her husband was going to prison — presumably on art fraud charges — and she planned to leave him and move to Washington, DC, court documents said.

Ana got “uncharacteristically emotional and extremely upset” when she confessed this to her pal during dinner, the papers said.

Brian’s attorney admitted Thursday that his mother had hired a private investigator to tail Ana during a trip to the nation’s capital because she suspected infidelity.

But her son had no reason to suspect his wife was cheating until she vanished, the attorney alleged.

Prosecutors disputed this, saying Brian obsessively checked the Instagram account of a Washington, DC man whom he thought was his wife’s love interest.

Prosecutors maintained Brian had a lot to gain if Ana died — like $2.7 million in life insurance payouts, according to the Daily Mail.

But the defense claimed that was no motive — he didn’t need the money because his family was already loaded. The defense argued Ana could have disappeared of her own accord.

Walshe has pleaded not guilty to murder, misleading police, obstruction of justice and improper conveyance of a human body.

He’s due back in court in August.