Tomas Ó Con Ceanainn, Lord of Uí Díarmata, died 1478.
Overview
Sub anno 1478, the Annals of the Four Masters state that
Thomas O'Concannon, Lord of Hy-Diarmada, was slain by the son of his own brother"
He was succeeded by William, who died later the same year.
References
Vol. 2 (AD 903–1171): edition and translation Annals of Ulster at CELT: Corpus of Electronic Texts at University College Cork Annals of Tigernach at CELT: Corpus of Electronic Texts at University College Cork Revised edition of McCarthy's synchronisms at Trinity College Dublin.