The Affair star had an intimate lunch with Lily from Downton Abbey in Rome and was pictured stroking her hair and cuddling and kissing the back of her neck.
The couple were also fooled around on a scooter while Dominic wrapped his arms around Lily as they bounced together.
Dominic has been married to Irish landscape architect Catherine FitzGerald since 2010 and they have four children together.
It is not known if the couple are still together, but no breakup has been publicly announced.
Lily is newly single after her on / off romance with Doctor Who star Matt Smith ended last week.
They had been together for about five years and tried to retry their relationship during the lockdown, but they have now broken up for good.
A source told The Sun, “Lily and Matt have been a perfect couple for years, but there are only a few differences they couldn’t overcome.
“They went back and forth this year to sort things out between them because they both love each other so much.
“But unfortunately it just doesn’t work and they mutually decided to close this chapter of their lives.”
Dominic and Catherine dated college and got married in Ireland in 2010. They have four children together, daughter Dora (13), son Senan (12), son Francis (11) and Christabel (seven).
The Wire star is also the father of another daughter, 22-year-old actress Martha, with ex-girlfriend Polly Astor.
In addition to his role in the hit police thriller, Dominic is known for his lead role in the hit HBO drama The Affair.
On the show, he played a writer who starts a passionate extramarital affair with a young waitress played by Ruth Wilson after meeting on vacation.
The critically acclaimed series has been running for five seasons.
Mirror Online has asked a Dominic representative for comment.
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