The four historical Irish provinces are not administrative divisions, and never were. Nowadays, though, they are sometimes used as convenient divisions of the country. From about the middle of the 17th century, coats of arms were attributed to the provinces. Flags representing the four provinces are now widely used, but there seems to be no evidence of their existence before the 20th century. There also exists the "Four Provinces Flag", which occasionally appears as a kind of substitute national flag. It consists of a flag composed of four quarters with the four provincial flags (clockwise, from upper hoist: Ulster, Munster, Connacht, Leinster). Source: Séamas Ó Brógáin: "The Flags of the Four Provinces".
Mario Fabretto, 21 August 1996 via CRW Flags