Just remember, your data roaming charges are capped only within the EU
! If you go to the States and use data, you can still be hosed.
Also, watch out for the mobile service they provide on the ferry. That's run by a Norwegian consortium, so it's not covered by the cap.
On the OP, if you don't pay your bill, they'll usually refer you to a collections company. Not paying your bills is not a criminal offence, though, except for the special case of not paying any bills the Government sends you ("It's good to be king", indeed).
If you've a dispute, ignoring the bill is not the way to go about resolving it. They could bring you to court, and your refusal to pay at all wouldn't look great.
Write to them (keep copies), explaining why you think the bill is too high, and always offer them an alternative way of you paying it (e.g. an extra €10 a month in future until it's all gone).
Finally, if this is question was because you're in trouble, have a look at http://mabs.ie/
. They've a free advice line too.