By Professor Brian Ó Cuiv, Celtica (1987)

(reprinted with the author's permission)

        In Lexique Étymologique de l'Irandais Ancien C-199, where the forms cossoit, cosait are given as verbal noun of con-saidi, we find as a derivative of this "cossaitech 'querreleur' ou 'qui se plainte' (d'ou NP O'Cassidy."  We may suppose that the compilers might postulate Ua Cossaitig as a likely form of the surname in the Middle Irish period.  This is theoretically possible since it would be on par with Ua Cleirig from an ancestor Cleirech.  The later reflex of such a form would O Cosaoidigh or, with a for o in the first syllable as seen in the later v.n. casaoid, Ó Casaoidigh.  However, extant forms of the surname do not, as far as I know, support the statement in the Lexique. 

        The forms found in modern times are Ó Caiside, which would be a reflex of a Mid. Ir. Ua Caisite, and Ó Casaide.  The form with palatalised s is the one generally used by a number of seventeenth-century O Caiside scribes and also in other early sources, such as annals, but in some instances the i-glide in the first syllable was omitted to read Casside or Caside.

        The Ui Chaiside were prominent from the fourteenth century on in Fermanagh where many of them were hereditary doctors to Mag Uidhir chieftans.  As is the case with many learned families their origins are obscure.  It seems likely that the connection with Fermanagh went back to the twelfth century. 

        A late genealogical tract traces the family through Cormac Cas, ancestor of Dal Cais, to Mil Easpainne (Anal. Hib. 3. 136-7).  The name Caiside occurs at two points in the genealogy:  eighth and fifteenth down Cormac Cas.  The earlier Caiside is given as great-grandson of Caisin (which probably for Caisin) "a quo an Fine."   The section showing the later one is as follows:  ". . . mc Ghiollamachuada mhoir mc Caiside, do bhi na ollamh leighis agus Seannchuis tir Chaidide."  

         It is likely that the "Giollamachuada" of the text is the Giolla Mo Dutu who composed the metical banshenchas in 1147 and who is described by Best and O'Brien in LL Vol. III, p. viii, as "Gilla Mo Dutu ua Casaide."   According to his own account in the closing stanzas of the poem he composed it in Daminis ( = Devinish on Lough Erne), but he had come from Ard Brecain ( = Ardbreccan in Meath).

        The name of the eponymous ancestor of the family is quite unusual.  It does not occur among the personal names listed in Corp. Gen. Hib.  The comment "a quo an Fine" following the name Caisin in the genealogy cited above and the fact that the line goes back to Cormac Cas suggest that the author of that genealogy saw a connection between Caiside (earlier Caisite), Caisin and Cas.  Also uncommon is the termination -ite, but it is seen in the name Tipraite from which came the surname Ó Tiobraide (Tubridy).  It is noteworthy that in an early genealogy of the Ui Chaisin(e) branch of the Dal Cais (Corp. Gen. Hib. p. 248) a Tipraite appears five generations down from Caisin.