She returned to Dublin at the beginning of WWII and joined the Irish fashion house of Richard Alan. She quickly rose through the ranks to become Director at the age of 22. From 1953, she designed her own couture line under her own name, Sybil Connolly for Richard Alan. This was launched at Dunsany Castle and Connolly was the star. Thus she became an over night success story and well known in Ireland, the USA and later Australia.

She left Richard Alan in 1957 and established her own fashion house. She gained much publicity and attention for her unusual adaptations of traditional textiles such as Irish linen, crochet and lace, and redesigning them into delicate blouses and dresses. 1967, Jackie Kennedy visited Ireland and Connolly and wore one of her linen pieces in an official White House portrait.

Throughout the 1950s and 1960s, Connolly helped popularise hand-woven woollens, tweeds, mohair as fashion fabrics. She also developed into interior design and gained a huge reputation, she also wrote books on Irish homes and designed crystal and pottery for Tiffany & Co.