The council maintained eleven seats and its members were known as "princes", regardless of gender. At the head of the council sat the High Prince (Se'har Maera), who was elected from the council to serve a life term. A High Prince reserved the right to step down, but would lose a seat on the council.
If a seat became vacant, candidates of the royal standing were nominated and voted upon by seated members. The process was not entirely democratic, as the High Prince maintained a vote worth five, but was unable to nominate a candidate.
"[...]Jack “the End” Daemon, tried to kill Conall Taylor, a sitting member of the Council of Princes in Tír Tairngire, about two weeks ago. [c. 2074] Apparently, the Tír authorities had been tipped off about the plot ahead of time and took precautions. Daemon took out the wrong target, and the Tír forces then took him out.[...]" -- Snopes