Thirteen. It’s an unlucky number for some, but certainly not for fans of metal. I say metal. Of course, what I really mean is METAAAAAAAAAAAAAAAAAAL! This year sees some of the biggest and loudest  bands in the history of metal descend upon Ireland like an eagle on a sleeping mouse. If you aren’t a big fan of heavy guitars, you might be safer indoors for most of this year. Don’t say I didn’t warn you. Bands such as Slayer, Mastadon, Nine Inch Nails, Lamb Of God and even the fathers of heavy metal (or should that be grandfathers) – Black…