In an upcoming episode of Who Do You Think You Are?, the Titanic star confirms a hunch her maternal ancestors were Swedish.
But she also finds out she had a great, great, great grandmother called Eliza O’Brien, who hailed from Dublin, on her dad’s side.
During filming of the show, she visits her father, Roger, in Reading, England, where she grew up.
She said: “On my Dad’s side all I really ever knew is there was lots of pubs, lots of booze clearly, lots of actors.
“But much more than that I don’t really know.”
When she begins digging back through the family records, she is astonished to find her Irish great, great, great grandmother, Eliza O’Brien, had 11 children.
She added: “My great, great grandmother Marian Colquhoun was one of 11 children. Her parents were William Colquhoun and Eliza O’Brien.”
The programme doesn’t explore the background to her great, great, great grandmother Eliza O’Brien, a military seamstress — possibly because there are few records on her life — but they do examine the life of her soldier husband, William Colquhoun, who is from Holborn in London but has an Irish surname.
His father is also named as William Colquhoun on the family tree but it isn’t known where he originally comes from as the records don’t survive for him or his distinctly Irish surname.
The actress is later told he was in the Grenadier guards, the regiment who guard Buckingham Palace.
She said: “I hope he was good at his job because he had 11 children and a young wife, a lots of mouths to feed.”
In the hit BBC genealogy show, Winslet characterises her upbringing in Reading as working-class, revealing: “My parents never had any money and it was always a real struggle but we were very, very happy.
“My dad, who was an actor, always had to get whatever odd job he could.”
During the show, Kate gets emotional as she reveals she was partly prompted to trace her roots by the tragic death of her mother Sally from ovarian cancer two years ago.
She said: “Mum absolutely loved children. That sense of family has carried on in all of us. My mum, Sally, died nearly two years ago.
“She had ovarian cancer, she was an amazing lady, she should still be alive.
“We all miss her terribly. She was very much the centre of the family, she didn’t talk about her family story very much.
“When we were younger there was a rumour about some Scandinavian thing.
“It’s something I’ve always wanted to know, particularly since my mum died.”
She later finds out her ancestors were in fact from Sweden but her delight turns to tears when she uncovers the extreme hardships her ancestors endured.
Her great, great grandfather Alfred Lidman became a tailor, who had moved over to Britain from Sweden.
Kate noted: “I really know where I came from and that to me is the most wonderful thing to come out of this story.”
Who Do You Think You Are? — Kate Winslet, will be shown on BBC One on Monday, August 12 at 9pm