Wiktionary unusually has no etymology for it, but it's fairly clearly derived from roth (wheel), which Wiktionary lists as from Proto-Celtic *roto- (wheel, chariot) < PIE *Hroth₂-o-, and rotate etc. are from L. rota (wheel); that points to Sanskrit रथ which is in turn listed as cognate with Old Irish roth. So I think your guess is probably right.