The decline of the domestic linen industry in Derry from the 1830s and the failure of a factory linen industry played a significant part in the birth of the shirt industry as there was a plentiful supply of female labour, skilled in working with linen, seeking employment. The sprigging industry (a form of embroidery) for a while gave employment to this workforce, but more importantly, for the future, it created a female labour force renowned for its skill in needlework.
Shirtmaking in Derry reached its peak in the 1920s when the shirt factories, together with their associated outworkers, employed 18 000 people. Derry was the principal seat of the shirt industry in the UK and, Derry city became not only the shirt supplier of the UK but also of Europe and the British colonies.
https://www.culturenorthernireland.org/features/heritage/shirt-industry