The Irish actress, who plays a young Irish immigrant, is widely tipped to get an Oscar nomination next year.
Picking up the award at Sunday night’s ceremony in London, Ronan revealed that she had met the film’s producers at the same awards two years ago.
In the film she plays Eilis Lacey, a young woman from rural Ireland in the 1950s, who has to leave her home to find job opportunities and a future in the US.
For Ronan herself the film was particularly personal. Her parents were migrants to the US in the 1980s and she was later born in New York.
“I got an amazing role to play. I was given the opportunity to honour a journey and a story that’s very close to me,” she said during her acceptance speech.
Speaking backstage, she said the film had achieved a “universality” since its release.
“A lot of people related to it because everyone’s left home and felt that way.”
She added that Irish actors and directors were having a great year. “It’s really important to represent home for me. I think we’ve got a nation of really terrific storytellers, we always have done, and that’s kind of our culture and our roots.”
Other acting winners included Tom Hardy who won best actor for his dual role as Ronnie and Reggie Kray in Legend.
Olivia Colman won her third Bifa for her best supporting actress performance in The Lobster. Brendan Gleeson won best supporting actor for Suffragette, having won last year’s best actor prize for Calvary.
Sci-fi thriller Ex Machina was the big winner of the night with four awards, including best British independent film. It also won best director and best screenplay for Alex Garland and outstanding achievement in craft for its visual effects.
Ex Machina producer Andrew Macdonald told the BBC backstage that the technology and the visual effects had allowed the film-makers to tell “a human story.”
Kidnap drama Room was named best international independent film. Jacob Tremblay, the nine-year-old star of the film, collected the award with director Lenny Abrahamson.
As previously announced, Kate Winslet received the Variety Award for helping “to focus the international spotlight on the UK”.
Picking up the award, the Oscar-winning actress said: “In spite of some Titanic moments, independent film is where I have learnt the most and laughed the most.”
The Richard Harris Award for outstanding contribution to British film was presented to Chiwetel Ejiofor.