For not only did the nomination of O'Neill's reputed son Matthew as his heir with the title of baron of Dungannon by the English king conflict with the Irish custom of tanistry which regulated the chieftainship of the Irish clans, but Matthew, if indeed he was O'Neill's son at all, was illegitimate; while Shane, Conn's eldest legitimate son, was not the man to submit tamely to any invasion of his rights.
Turlough had been elected tanist (see Tanistry) when his cousin Shane was inaugurated the O'Neill, and he schemed to supplant him in the higher dignity during Shane's absence in London.
Two legal decisions swept away the customs of tanistry and of Irish gavelkind, and the English land system was violently [From Anglo-Norman Invasion] substituted.
TANISTRY (from Gaelic tana, lordship), a custom among various Celtic tribes, by which the king or chief of the clan was chosen from among the heads of the septs and elected by them in full assembly.
Tanistry was abolished by a legal decision in the reign of James I.