Ancient Celtic Symbols of Protection

[Prue's Doux]

On the History Channel, I love this program called "Cities of the
Underworld". One episode, "The City of Blood", focused on London.
They covered Bedlam's Bank, a subterranian rock quarry 40 miles
outside of London, which was in full operation by 1200 & abandoned in
the 17th century. In the 16th portion of the tunnels are found chalk
marks of pre-Christian Celtic symbols, which were used to invoke
protections from the earth spirits, ward off evil spirits, & warn
others that they are near the mouth of Hell. The use of these symbols
were banned by the Church as a practice of witchcraft, but so great
was the fear of being underground that they risked the death penalty
for what they feared was something far worse.

I'm interested in symbols & alphabets & use many in my art & writing.
So I would like to see the system of these. They are called what
sounded like "potter-pay-ick", which I assumed is spelled
"paterpaic". But I found, in all search engines, this word doesn't
exist. I only heard it mentioned once so I may have heard it
incorrectly. Does this sound familiar to you? Have you ever heard of
anything like this? Where I can I find information on this?