There are 220 Cassidy voters registered in Fermanagh today , which leaves the name the thirteenth most common name in the county and the fifth among the native Gaelic families. For over a thousand years the Cassidys were very prominent in Fermanagh in the fields of literature, medicine and religion. Despite this we are not sure of their origins. Genealogies present them as Munster Dalcassians. Whether they came from the deep south or not we do not know. Possibly they came here in the wake of Brian Boru's army. More probably they were an old Fermanagh family or an Oriel family who had a Munster genealogy invented for themselves during Brian Boru's reign, when it must have been fashionable to be from the south. At any rate, the family lived at Ballycassidy, near Enniskillen. Like other professional families, they moved out to other parts of Fermanagh and indeed to other parts of the north, and their fame as doctors and scholars spread throughout all Ireland.
Even before the Maquires assumed control of Fermanagh, Giolla Mochuda Mór Ó Caiside was famed throughout Ireland as a prince of learning. He flourished from 1130 to 1140 and his Gaelic verse is still read. Equally famous was the Penal Day poet, Tomás Ó Caiside. Archdeacon Rory Cassidy helped with the Annals of Ulster and he compiled the Clogher Register in 1525.
The Cassidys were also herenachs at Devenish and they supplied the Church with many clerics. Most of these laboured during the Penal Days and some of them were Franciscans. Father Ó Gallachair has given us an account of these in the Clogher Register of 1956.
Despite the limited franchise there were fourteen Cassidy voters in Fermanagh in 1796. Perhaps the most famous Cassidy in the last century was William (1815-73), the great American Catholic politician.
from The Fermanagh Story by Pedar Livingstone; 1969
Here are some translations and definitions of some words and phrases used above:
Herenachs: a family with an hereditary title to Church property.
Penal Days: 1690-1750 Laws enacted in Ulster which essentially deprived all adherents to the Catholic faith of their civil rights.
Motto: Frangas non flectes.... meaning literally "broken not bowed". A less literal translation, used by the Cassidy-L mailing list, is "You can break me, but I'll never bend."