Lady Wilde, Ancient Legends of Ireland, pages 48-49.

he islanders, like all the Irish, believe that the fairies are
                 the fallen angels who were cast down by the Lord God out
                 of heaven for their sinful pride. And some fell into the sea,
                 and some on the dry land, and some fell deep down into
     hell, and the devil gives to these knowledge and power, and sends
     them on earth where they work much evil. But the fairies of the
     earth and the sea are mostly gentle and beautiful creatures, who
     will do no harm if they are let alone, and allowed to dance on the
     fairy raths in the moonlight to their own sweet music, undisturbed
     by the presence of mortals. As a rule, the people look on fire as the
     great preservative against witchcraft, for the devil has no power
     except in the dark. So they put a live coal under the churn, and
     they wave a lighted wisp of straw above the cow's head if the beast
     seems sickly. But as to the pigs, they take no trouble, for they say
     the devil has no longer any power over them now. When they light a
     candle they cross themselves, because the evil spirits are then
     clearing out of the house in fear of the light. Fire and Holy Water
     they hold to be sacred, and are powerful; and the best safeguard
     against all things evil, and the surest test in case of suspected
     witchcraft.