According to Cassidy, the English word sneak comes from the Irish snighim. He also claims that the Barnhart Dictionary of English Etymology concurs. I haven’t got a copy of Barnhart’s and I can’t be bothered to go and look for one, but this claim seems unlikely, given that all the other dictionaries trace it, quite logically, to the Old English snican which is related to cognates in the other Germanic languages as well as to the root of the English word snake. As usual, Cassidy is being economical with the truth here. Snighim is also an unsuitable source for a word used of people, anyway, because it is used for slow-moving animals like snails and slugs. It doesn’t mean to move sneakily or furtively. However, in any case, it has an impeccable Germanic origin and so it can’t come from Irish. As usual, it’s total nonsense.