It is pefectly legal to fly on top of an overcast on an IAA issued JAR PPL, in Ireland or the UK (the two that you've asked about). You must of course comply with the VFR cloud sepearation and visibility rules appropriate to the class of airspace that you are operating in.
You must of course be able to comply with these rules while getting up there, and when getting back down again.
Of course whether this is sensible, and when it is sensible and when it is not is a whole other area of discussion. Having no instrument experience or a poorly equipped aircraft, and no way to get back down would not be a nice experience!
The situation for a UK CAA issued PPL is somewhat different. The CAA issue a restriction on a UK PPL which requires the holder to be in sight of the surface at all times. If the UK holder gets a IR or a IMC rating, then this restriction is removed from their licence.