A ten-minute stroll from Spencer Dock, across the Liffey and along the low-rise, glass offices of Dublin’s docklands, separates the world’s second-largest professional services firm, PwC, from rivals Grant Thornton. This is Ireland’s financial district; unimposing for the most part. Yet contained within this half-mile stretch is a negligence lawsuit worth €900 million pitting the two accountancy giants against each other. It is the biggest and among the most complex damages claims to come before the Irish courts, involving a pool of some 40 million documents. The litigation costs alone could top €60 million. The genesis of the dispute is…