I gather this is more of a 'featured extra' thing than any more substantial role. It seems to be in thanks for Lightbody and his bandmates being huge fans of the show and frequently mentioning it on their webpages. During the filming of the previous two seasons, the band (half of whom are from Northern Ireland and sometimes use Belfast as a base of operations) hung out with castmembers and crew on several occasions.
Lightbody with producers D.B. Weiss and David Benioff in Belfast.
Interesting news. Thrones still has a very low profile in the UK, due to its transmission on a satellite channel with a tiny audience (although Thrones is by far the most popular show to ever air on it, it's still very obscure here). This has changed a little bit as more magazines pick up on it and as the DVDs and Blu-Rays continue to sell excellently. BBC Radio 1 also has featured the show several times recently as a recommended show, and in fact one DJ got into a mock-Twitter feud with Lightbody when the latter accidentally spoiled a plot twist for him. If this kind of stunt-casting can help raise the profile of the show a little over here, I'm all for it (even if at one stage I feared madness induced by hearing 'Chasing Cars' six thousand times a day, every day on the radio).