As mentioned in the Luas BXD thread, the tender for the bridge as been awarded to Graham Projects Limited. Graham have been involved with other high profile bridge projects before.
From Dublin City Council's page on the project:
Dublin City Council is planning to build a new bridge over the River Liffey in Dublin City Centre, just downstream of O’Connell Bridge. The bridge will carry buses and the LUAS at this critical location across the Liffey. It will have cycleways and generous footpaths that will open Marlborough Street and Hawkins Street to greater footfall, prospective development and create a new north-south link from Abbey Street to Pearse Street.
The bridge will have an elegant contemporary design. It will be a slender, single span, smooth concrete structure, with the underside of the bridge being designed to be as high above the water as possible so that river traffic is not impeded.
Construction on the bridge is due to commence in Autumn 2011. It will take approximately 18 months to build.
More detailed draws of the above and other elements can also be found in this PDF here.