Saoirse Ronan is reportedly dating Jack Lowden.

The 24-year-old actress is said to have struck up a romance with her 'Mary Queen of Scots' co-star but others working on the movie had no idea they were together until a screening of the movie in New York on Tuesday (04.12.18).

A source told the New York Post newspaper's Page Six column: ''We don't know when they got together, we only found out on Tuesday night when they rolled up together -- but they seem very happy and sweet.''

After the screening, the 'Lady Bird' actres and the 28-year-old actor looked very close at the afterparty at Tavern on the Green.

An insider observed: ''Jack was following her all over. He even followed her to the bathroom and was waiting outside for her. They were very close.''

The pair were joined at the bash by Saoirse's mother Monica, but the group had to cut short their celebrations as the actress had to fly back to the set of 'Little Women'.

Before they started shooting the film, Jack said he was excited to work with the 'Brooklyn' actress.

He gushed: ''She's lovely . . . I can't wait for that it's going to be fun.''

Saoirse was previously believed to be dating the 'Dunkirk' actor's brother Calum, a ballet dancer, after they met at the BAFTAs earlier this year, but she has been increasingly spotted with Jack in recent months, including on a vacation to the Dominican Republic.

The 'Lovely Bones' actress has previously admitted she isn't very good at dating because she doesn't know what to do.

She said: ''I don't know how to date ... I don't date, I'm not into the whole, 'Let's go out for dinner.' ''