Born in New York City, Ronan is the only child of Irish parents Monica and Paul Ronan – the latter an actor – who were living in New York at the time. She was raised in County Carlow, Ireland, having moved there when she was three years old. Ronan is currently home schooled and lives in County Carlow. As a baby, she sometimes accompanied her father on the set when he was working on such films as The Devil's Own.
Ronan supports the Irish Blue Cross, as the charity had reunited her with her runaway dog Sassie. On her name, she said, "'Searsha' is how Irish people pronounce my first name, but I would pronounce it 'Sersha,' like 'inertia'. It's Irish for 'freedom.' I recently found out that my middle name, Una, means 'unity' in Ireland. And I think my last name means 'seal.' So I’m a free, unified seal."