Saint Patrick's Saltire

Saint Patrick's Saltire or Saint Patrick's Cross is a red saltire (X-shaped cross) on a white field, used to represent the island of Ireland or Saint Patrick, the patron saint of Ireland. In heraldic language, it may be blazoned Argent, a saltire gules. Saint Patrick's Flag (Bratach Naomh Pádraig) is a flag composed of Saint Patrick's Saltire.

The red saltire's association with Saint Patrick dates from the 1780s, when the Order of Saint Patrick adopted it as an emblem. This was a British chivalric order established in 1783 by George III. There is some evidence that a similar saltire was occasionally used to represent Ireland before this. It is often suggested that it derives from the arms of the powerful Geraldine or FitzGerald dynasty. Most Irish nationalists reject its use to represent Ireland as a "British invention".

After its adoption by the Order of Saint Patrick, it began to be used by other institutions. When the 1800 Act of Union joined the Kingdom of Ireland with the Kingdom of Great Britain, the saltire was added to the British flag to form the Union Flag still used by the United Kingdom. The saltire has occasionally served unofficially to represent Northern Ireland and been considered less contentious than other flags flown there.

...
Wikipedia