Wouldn't call them a heavy criminal presence in Belgium and the Netherlands these days. A few years ago Irish Travellers did show up in Belgium getting involved mainly in their trademark roofing scams. Not so much nowadays. Irish Travellers still show up whenever there's a major horse event going on though, but they aren't really a major bother there as far as I know.

Slovak and, to a lesser extent, Romanian and Bulgarian Roma gypsies are a bigger pain in the ass over here. Squatting, pickpocketing, petty drug dealing...
You're far more likely to experience nuisance from the Slovak Roma than from an Irish Traveller, a Dutch "woonwagenbewoner" (who are mostly pretty much alright in my experience) or even a Roma gypsy from former Yugoslavia.