Yeats, Celtic Twilight - "A load of old tosh/tais"

Preparing for a lecture on Yeats, early poetry and critical essays for Wien students, and came across Robert Welch – who died too young – and his essay for the Penguin edition of Yeats Folklore, Legend and Myth (1993). His intro is fizzing with ideas and the glossary at the back I am copying for the Austrian students in my classes. What struck me was (on p.455 if you are looking this up) a reference to Tash: (Tosh) Ghosts,  which Welch cites as Yeats correctly taking from the Gaelic tais (singular).

It struck me that English usage has the phrase ‘what a load of old tosh’ meaning stuff that is unbelievable, nonsense, with no material basis. My chambers has it as ‘bosh, twaddle’ and a secondary meaning being neat/tidy – from Scottish.

Could this be the case of an Irish word that has silently been absorbed into the English language with the vestiges of its Gaelic meaning?