Yes, they really do tattoos at the convention, I have had several friends get one done there with no issues. There is a website for the convention: http://www.tattooexponapoli.it/en/ and they do have listings of the tattoo artists and their websites.
They have a convention every year and there is one in rome as well. I know of several tattoo artist in the area which have their own shop and do very good work. Anima Sotto Pelle, in Giugliano, i buy my piercing jewelry from them, they do speak english ok. And Vitu Tattoo in Gricignano, who does not speak much english, but i have seen their work and it is very good.
Prices as always vary by artist and what you are intending to have done, but i can say that they are comparable to what you would pay any where else.