Gwynn was born Eirwen Meiriona St John Williams on 1 December 1916 at 99 Shiel Road, Newsham Park, Liverpool ; her father wrote her date of birth as 12 December to avoid a fine for late registration of birth. and was disappointed his first born was a girl. She was the eldest child of the self-trained dentist William St John Williams and his wife Annie (née Williams); the family resided amongst a large Welsh diaspora in Liverpool. Gwynn had a younger brother, Gwilym Gareth. Her father encouraged her to become interested in current affairs, read widely and become proud of her family roots in North West Wales. From 1923 to 1927, Gwynn attended Birchfield Road Council School, and enjoyed on-stage performing. She learnt to resist bullying from her fellow pupils and teachers because she was Welsh.