Which Welsh ironworks became the world's largest ironworks in the early 19th century?

The Welsh ironworks that became the world's largest in the early 19th century was Cyfarthfa Ironworks.

Located near Merthyr Tydfil in South Wales, Cyfarthfa began in 1765 and expanded dramatically under ironmaster Richard Crawshay, who took control in the 1780s. Its position near coal, iron ore, limestone, waterpower, and transport routes helped make the Merthyr district a major centre of iron production.

By the early 19th century, the works had several blast furnaces and rolling facilities and employed thousands of people. It adopted methods such as Henry Cort's puddling process, allowing pig iron to be refined into wrought iron. Cyfarthfa produced rails, bar iron, and other products for Britain's rapidly expanding industrial economy.

The answer is sometimes confused with Dowlais Ironworks, another giant works in Merthyr Tydfil. Both were exceptionally productive, and Dowlais later became the larger operation at certain points. But for the wording about the early 19th century and the world's largest ironworks, Cyfarthfa is the standard answer.