Nicky, I was responding to the Original Poster, whose income was below the threshold at which he would need to pay Prsi. Voluntary Prsi in that case is €253.
Believe it or not, there are many of us self employed who are working for amounts you would probably turn your nose up at, but we soldier on somehow, hoping to qualify for the State pension. Although it seems to be getting further away, not closer.
idontknow, you won't lose the benefit of your previous years' contributions, but missing a year will lower your average.
Also, if you have no contribution for 2010, it would affect your entitlement to a benefit that looks to 2010 as the qualifying year. Maternity benefit, adoptive benefit, bereavement grant eligibility, all refer to contributions made in a particular year.
You know your situation, and whether you may be likely to require any of those benefits in 2012. If so, you might regret having made the small saving on Prsi.
By the way, dental benefit is not a Class S benefit.