The Oscar-nominated film, starring Judi Dench, is based on a true story about an Irish mother searching for her American-raised son years after being forced to give him up at birth – and it hit close to home for Deasy.
"I identified so strongly," the 59-year-old says. "I know that pain and separation. I was a banished baby."
Like the titular Philomena Lee, Deasy's mother – an Irishwoman named Johanna Sheehy – wasn't married when she got pregnant in 1954, a dangerous situation to be in at the time. Thousands of unwed mothers were deemed "fallen" by Ireland's Catholic Church and forced to live and work in convents or Magdalene Laundries under the watch of nuns.
As for their children, they were often taken away at birth and put up for adoption.
Deasy was raised by nuns in Ireland until the age of 4, when she was put on a plane to New York and thrust into the arms of a new American family. In this week's PEOPLE, she recounts her own heartbreaking story of trying to find the mother she never knew.
Amazingly, after she spent decades searching, Deasy found her birth mom, and the two reunited for the first time in 44 years.
"I had to overcome many obstacles to put together the missing pieces of my life," says Deasy, "But all my questions were answered the day I laid eyes on my birth mom. I found my truth."
Watch a clip of Deasy's emotional journey above, and pick up this week's issue of PEOPLE, on newsstands Friday, for the full story