From our friends at i09:
It was announced he was going to be in the show, and he said that he’d finished his filming in a few days in Belfast, but we didn’t know who he was cast as until now.
Winteriscoming.net broke the news today that Mark will be playing Tycho Nestoris, a representative of the Iron Bank of Braavos. His description as ‘a tall, gaunt stick of a man with long legs and a beard as thin as a rope from his chin and almost reaching his waist’ sounds like Gatiss. I can’t wait to see what that beard looks like on him.
The character first appears in A Dance with Dragons, the fifth book, so he’s being introduced earlier as the show’s third and fourth seasons are (roughly) adapting A Storm of Swords, book three in the series. There was another Bravosi banker with a small role in ASOS, though, so it seems likely the two are being merged.