ceithre and ceathra
Definitions (by source)
(adj.) four; the two forms are used indifferently, 17, 26, 27; the initial is gen. aspirated except when this is prevented by the preceeding word, 213, 1748, 1811, 2503
Trí Bior-Ghaoithe an Bháis: The Three Shafts of Death. Editor: Osborn Bergin.