St. Patrick’s Flag (since the 19th century)
If you know much about flags, you probably know that the Union Jack is a combination of the St. George’s cross of England, the St. Andrew’s saltire of Scotland, and the St. Patrick’s saltire of Ireland. What you might not have known is that the St. Patrick’s flag was practically invented for the occasion.
The first organization to associate the red diagonal cross on white with Ireland’s patron saint was the Order of St. Patrick, established in 1783 as an Irish counterpart to the English Order of the Garter and the Scottish Order of the Thistle. Before then the cross associated with St. Patrick was usually upright, and was often a cross pattée (a cross with flared arms). The colours could vary but green and red (sometimes on a yellow background) were common.
Contemporary Irish opinion was against the Order’s badge, saying that their saltire was too Scottish. But it was this version of St. Patrick’s cross that made its way onto the new Union Jack in 1801, and it was only after then that it began to used as a flag on its own. There’s some evidence of red saltires on white being used in Ireland before 1783, but none of them have ever been definitively linked to Saint Patrick or Ireland itself.