The problem comes from the mindless few. The ritual leavings (?) of people are bad enough, but take The Mound Of Hostages on the Hill of Tara. Like many passage tombs it has a gate. When you look inside you can see someone has carved their initials right over a cup + several rings on the only decorated orthostat there.

What I will say though id the railings do not deter the determined vandal. Ballybrack dolmen was protected once but it didn't stop someone 'tagging' it with red spray paint. In fact I think the railings probably made the act more likely; more of a challenge.

There is no excuse for ritual tosh being left behind though.