Which saint's red diagonal saltire is part of the Union Jack?

The red diagonal saltire in the Union Jack represents Saint Patrick.

It was added in 1801 when the Kingdom of Great Britain and the Kingdom of Ireland were united to form the United Kingdom of Great Britain and Ireland. The red saltire is conventionally identified as Saint Patrick’s Cross, symbolising Ireland in the composite flag.

The Union Jack combines three heraldic designs: Saint George’s red cross for England, Saint Andrew’s white saltire for Scotland, and Saint Patrick’s red saltire for Ireland. White edging, called fimbriation, separates the red elements from the blue field and helps preserve heraldic contrast.

A common visual trap is Saint Andrew’s saltire, which is also diagonal but white rather than red. Saint David, the patron saint of Wales, does not appear in the Union Jack because Wales was treated as part of the Kingdom of England when the original union-flag traditions developed. The modern design dates from 1 January 1801.