The Diocese of Clogher contains most of County Fermanagh, all of Monaghan, parts of Donegal, Tyrone, and a small piece of Louth. With the transformation of the Irish church in 12th century from a monastic church to one based on a diocesan and parochial system, church lands vested in the local bishops were accorded many of the privileges of the clergy. In return for continuing possession of the lands, the erenaghs paid the bishop an annual rent, many specific tributes and services, including a night's lodging and entertainment for the bishop and his train, helped maintain the local church and were obligated to maintain hospitality for "pilgrams, strangers and poor travelers." The members of erenagh families enjoyed a quasi-clerical status, and a greater part of the clergy was recruited from these families.
Cassidy - Ó Cassidy - Ó Caiside is an ancient Irish name. In the United States, many Cassidy variations exist, including Cassity, Cassedy and Casada.All Cassidys originate from County Fermanagh, North Ireland, where for centuries they were prominent in the arts and fields of medicine and religion. Like other professional families, over time they moved out to other parts of Ulster province and all of Ireland. The most important early Irish Cassidy priest and scholar was Ruaidhri Ó Caiside (Rory Ó Cassidy, died 1541), the archdeacon of Clogher. In 1525, Ó Caiside compiled the "Register of the Diocese of Clogher." He also carried on the work of Cathal Óg MacManus, who died in 1498, in the compilation of the "Annals of Ulster."
There is a dispute as to whether they belonged to the Fir Manaigh, the first Celtic settlers of County Fermanagh, or migrated to the area in the sixth century A.D. during the time of St. Molaise who founded the monastery at Devenish Island. Ui Caiside was closely linked to the monastery and had access to its school at a time when education was denied to most. The Ui Caiside served as hereditary physicians to the Mac Uidhir (Maguire of Fermanagh), the chieftains of Fermanagh. Cassidys are recorded as physicians to the Maguires between 1300 and 1600, and were also found practicing in the midlands of Ireland. From the very earliest times the Irish physician was attached to the clan or house of a chieftain. Thus, there were the Ó Cassidys of Fermanagh, the Ó Callenans of Desmond, the Ó Lees of Connaught among the leading Irish medical families. In the 16th century it was more typical for Irish doctors to study in schools established in Ireland or Scotland. From this period, Cassidys are identified as the authors of many medical tracts and as priests and scholars in the Diocese of Clogher, especially during the suppression of the Roman Catholic Church under the Penal Laws of the 18th Century. The contribution of the Cassidys to the priesthood originated from their position as one of the hereditary "church families" in County Fermanagh. Prior to 1600, large parts of Ireland, including one-sixth of the county in the Maguire period, were owned by the church. Local families, however, possessed the estates surrounding the churches and monasteries. The chief of the family bore the title of "erenagh." A single church with extensive lands might have several erenaghs, each erenagh controlling a separate part of the church's lands.
The earliest renowned Cassidy poet was Giolla Mochuda Mor Ó Caiside (also identified as Gilla Mo Dutu ua Casaide). Before the Maguires assumed control of Fermanagh, Ó Caiside was famed among the lir léighinn, the men of learning of Ireland "for all that was superior in Gaelic literature." Ó Caiside wrote in 1147 the poem Banshenchas (The Lore of Woman), which tells the history of women of the world. Former President Bill Clinton is a Cassidy, as his mother's maiden name was Virginia Dell Cassidy. President Clinton has Irish ancestry from both parents.