One of the most successful linen companies in the Strabane area was Herdman’s. Two brothers from Belfast, wishing to expand took a lease on an existing corn mill on the site in 1835 and by the 1850’s things were going so well that the prominent Belfast architect William Henry Lynn was employed to build a new mill nearby with a fashionable ‘Italianate’ tower. The tall brick chimney was built in 1877. In a 1919 hydroelectric power station was installed on site. The name ‘Sion’ derives from the Gaelic word ‘Sidhean’, meaning a fairy hill.