No penalty unless you file your tax return late. You may be hit for interest at the daily rate that equtes to c.10% per year but this really depends on the interest that would be due. My understanding is that the Revenue have to collect the interest manually with a letter and follow up enforcement action, If necessary, so they have a threshold level but I don't think they disclose.
I have seen late payments before, but never seen interest charges raised.
However, in the current economic climate, the Revenue may of course follow up on these matters more rigorously now.