Dermot Mulroney is remembering the late Diane Keaton, who played his onscreen mom in 2005’s “The Family Stone.”

Keaton died in Los Angeles on Oct. 11 at the age of 79. Her cause of death was later revealed to be pneumonia.

“It broke us all up,” Mulroney, 62, exclusively told The Post while promoting his new western “Long Shadows.”

“I had lunch with Tom Bezucha, the director of ‘The Family Stone,’ the week before. So we spoke so much about Diane,” he recalled. “Even then, I didn’t know that she was going to go so soon. She’d just been on my lips a few days before. So it really impacted me. And I was in touch with him and a few other people around that film.”

Released in 2005, “The Family Stone” featured an ensemble cast made up of Keaton (Sybil Stone), Mulroney (Everett Stone), Cr

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