This is commonly referred to as a St. Patrick’s Cross, but it is not. It was the arms of the FitzGeralds, Dukes of Leinster, a leading Anglo Irish family, and it was first used in a national context in the insignia of the Order of St. Patrick in 1783.

In 1782 Britain acknowledged the exclusive right of the Irish parliament to legislate for Ireland. To reflect the country's enhanced constitutional status, an order of chivalry called the Order of St Patrick was established in the following year.

After the union with Britain in 1801, the St Patrick's Cross continued to feature in the arms and flags adopted by various professional and public bodies during the nineteenth century.