Hahaha, in Hiberno-English, 'deadly' is synonymous with the US-English 'cool', or 'fun', or in HIberno-English, 'great craic', so I had to read your comment a few times to understand what you meant by that!
Though I've never tried hanging out inside a fridge, so maybe it is great craic after all!
I'm from Louisiana, and people are in the fridge down here. I'll yell at my kids to, "Get out of the fridge!", meaning "quit standing there with the door open because you're letting all the cold out." I've even complained about them "being in the fridge all day" during the summer.
Maybe it was this fear: http://www.amazon.com/Round-Ireland-Fridge-Tony-Hawks/dp/0312274920
After reading this, I think this might be what they say when someone is standing there with the door open, pondering the plethora of choices before them, being indecisive about what they should eat. I don't think it's literally a person physically occupying the same space, but rather an expression. I hear this in English a lot too. My friends tell their kids "get out of the fridge!" constantly. But they aren't literally IN the fridge, they're looking inside for food.
What is the difference between "sa" and "san"? In the tips & notes, it says "Lenition occurs after the words ... sa/san in the". I think I am confused because comments on other sentences mentioned that "den" means "of/off the" because it is de (of/off) + an (the). I guess, following that, I was just expecting sa to mean "in" and san to be "in the", but it looks like both sa and san mean "in the"? Is it a singular/plural thing?
Yes, they both mean "in the", but sa is used when the next noun begins with a consonant, san is used when the next noun starts with a vowel. (That's to ease pronunciation)
You can look them up in a dictionary and memorize them, or go by instinct. The gender matter in Irish isn't as intense as it is in French, so I think it possible to proceed without memorizing noun genders, or thinking about them.