Most people dont eat the casing... if it was served to you in a restaurant, it would probably not have the casing on it anyway.
It is indeed a stuffed sheep's stomach... stuffed with oatmeal, onions, spices... and liver, kidneys, heart and lungs of the sheep.
Honestly, you could not eat it if you thought about what was in it, but as a once-or-twice a year thing, it's not bad. When i eat it, i generally eat it as a stuffing... like a chicken breast stuffed with it. Occasionally i might have the chip shop version, which is a sausage stuffed with haggis, battered and fried, with chips.