Dia 's Muire dhuit.
I'm pretty sure that they are referring to an
Alp Luachra, which is a small newt that crawls into a person's mouth if they fall asleep near water. It goes into the person's stomach and eats all of the food that they eat; so no matter how much the person eats, they stay skinny and start to waste away.
The traditional cure for getting rid of an
Alp luachra is to eat lots of salty, dry meat without drinking any water, and then lie down near water (usually a stream). The
Alp luachra will get very thirsty and crawl out of the person's mouth and jump into the water – it works every time!
Of course, this has nothing to do with a
púca or with the word
púca. In fact, there's really nothing supernatural about it at all, according to folklore.
There is an old traditional story about an
Alp Luachra in "Beside the Fire" by Douglas Hyde
Here's a link for ya:
http://www.sacred-texts.com/neu/celt/btf/btf07.htmHappy reading!