The term is both singular and plural, i.e. the plural of ironworks is ironworks.
From the en.wikipedia.org
The Bell brothers opened their great ironworks on the banks of the Tees in 1853.
From the en.wikipedia.org
The ironworks finally closed in 1963, still under the control of the Lees family.
From the en.wikipedia.org
The AA's membership was concentrated in ironworks west of the Allegheny Mountains.
From the en.wikipedia.org
Exhibitor Mike Tripp, of Doxey, Stafford, was selling his miniature ironworks.
From the expressandstar.com
The statue was cast near Paris, France in the Durenne ironworks in the early 1860's.
From the charlotteobserver.com
Numerous small industrial centres sprang up, focused on ironworks, using local coal.
From the en.wikipedia.org
Albany's main exports at the time were beer, lumber, published works, and ironworks.
From the en.wikipedia.org
It is probable that he also had ironworks at Coalbrookdale, but evidence is lacking.
From the en.wikipedia.org
More examples
The workplace where iron is smelted or where iron goods are made
An ironworks or iron works is a building or site where iron is smelted and where heavy iron and/or steel products are made. The term is both singular and plural, i.e. the plural of ironworks is ironworks.
Ironworks is a small independent record label created and owned by the lead actor of the Fox TV hit series 24, Kiefer Sutherland, and his long time friend and fellow musician, Jude Cole.
A collective name that describes a blast furnace, forge, mill, foundry, and/or specialty shop.