The National Library of Ireland has recently released 34,000 of its 630,000 original Irish photographs on its website.
They are available free to view as low resolution images, with the opportunity to order higher resolution images for a fee.
The photographs, which relate to 1860-1954, include a large number of churches and other religious subjects.
See: http://digital.nli.ie/cdm4/index_glassplates.php?CISOROOT=/glassplates
Searching for “mill” produces 70 hits and most are relevant.
The free images are adequate for many purposes and the higher resolution ones cost roughly the same as those from the Mills Archive. A nice source of rare images from our near neighbours