The Cavan native appeared in a David Koma black sequinned gown, which featured cut-out mesh panelling that was paired with jewels by Chopard.
Saoirse’s hairstylist Adir Abergel shared photos of the pair getting ready for the event on Instagram.
He captioned the post: “Back with my baby SAOIRSE RONAN tonight celebrating @jack.lowden as @chopard honoree at @festivaldecannes.”
The 28-year-old was attending the event at the Carlton Beach hotel where Jack had been honoured with a Chopard Trophy.
Every year, the trophy is awarded to two young actors at the film festival, in order to recognise and encourage their career in the film industry.
This year, the prize went to the Ugandan-British actress, singer, composer, and playwright, Sheila Atim and Scottish actor Jack.
Saoirse who is notoriously private when it comes to her love life, was first linked to Jack in 2018, when they played on-screen lovers in ‘Mary Queen Of Scots’.
However, she previously shut down rumours they were dating and during an interview with The Daily Telegraph in December 2019, when asked if she was dating Jack, the Irish star replied, “Noooo.”
When pressed by the reporter who said, "I’m not allowed to ask you or you’re not going out with him?” the actress responded, “Just no.”
Jack (31) is best known for his roles in Dunkirk, Mary Queen of Scots and Fighting With My Family.
Thursday's prize giving comes after a number of stars of the showbiz world were in attendance on the opening night of Cannes Film Festival on Tuesday.
After Cannes was entirely cancelled in 2020 and held under strict health protocols in 2021, the red carpet returned in all its glamour on Tuesday night for the Opening Ceremony.
Last week it was revealed that Saoirse Ronan and Dua Lipa were rumoured to have joined the cast of Greta Gerwig’s upcoming Barbie film.
While not a lot is known about the film just yet, fans got a first look at Margot Robbie in character as Barbie at CinemaCon last month, and it was revealed that Ryan Gosling will be playing her partner Ken.
The star-studded cast also includes America Ferrera, Kate McKinnon, Michael Cera, and Will Ferrell.
And at the Cannes Film Festival this week, word on the street is that Saoirse, who previously starred in Gerwig’s flicks Lady Bird and Little Women, and Dua, who will make her big screen debut in spy thriller Argylle later this year, are reportedly the latest additions to the film’s roster.
“Dua is making waves in both music and movies,” one insider said. “She really impressed in Argylle and word spread around Hollywood about how good she was.
“Now she has joined the cast of Barbie alongside big names including Margot, Ryan Gosling, Will Ferrell and Saoirse Ronan.
“Although Argylle is a big film in its own right, Barbie will be a breakthrough part for Dua because there is so much hype around it.
“Dua is quickly proving there is far more to her than just being a pop star. She has an exciting future ahead of her in films.”
But Saoirse and Dua aren’t the only celebrities who are reportedly lined up to join the Barbie cast, as Kyle Buchanan of the New York Times has been sharing some of the “incredible” gossip about the romantic comedy that he overheard at Cannes.
He explained that the festival has been bubbling with rumours that Marvel actor Simu Liu and Sex Education star Ncuti Gatwa will be playing alternate versions of Ken in the film, which hits cinemas July 21, 2023.
And Emmy nominee Issa Rae and Transparent star Hari Nef are reportedly set to play different versions of Barbie.
“I am hearing incredible Barbie goss at Cannes,” Buchanan tweeted on Wednesday.
“For one, I’m hearing that Ryan Gosling is not the only Ken in the Barbie film. Simu Liu and Ncuti Gatwa also play Kens.
“And, by the same token, Margot Robbie is not the film’s only Barbie. Issa Rae and Hari Nef play different Barbies…”
Details of Barbie's plot remain unknown, but it’s rumoured that the film will follow a doll who’s kicked out of Barbie Land for not being perfect enough.