Taoiseach is the term for Ireland's head of government. In English it's pronounced something like /ˡtiʃəx/. McBain's spells it toiseach "a beginning, a chief" and has no entry for taoiseach; I don't know why. Anyway, it's cognate with Scots Gaelic tòiseach and Welsh tywysog. It's thought to be from Proto-Celtic *to-vessiko-s "chief, leader", which according to Pokorny is from Proto-Indo-European *weid- "to see, know" (altho others connect it with a different root, *wedʰ- "to lead"). Other reflexes of *weid- include druid and Veda.