The Railway Procurement Agency is to offer a €150,000 prize to design a new bridge over Liffey Valley for Metro West, due to be open by 2014.
The bridge, which will display the "highest levels of design excellence", will carry the light rail system across the valley and must blend with the countryside.
The RPA said yesterday that some of the world's top architects had already expressed an interest in designing the bridge which will run almost parallel with the M50 motorway.
Metro West will run from Tallaght to Dublin Airport and link with Metro North at the Metropark stop, north of the M50.
The proposed route crosses the Liffey Valley, which is an area renowned for its wealth of ecological, archaeological and high amenity landscape value, the RPA said.
"This new Liffey Bridge for Metro West should be designed to make a prominent, yet still aesthetically pleasing addition to the Liffey Valley whilst enabling views across the Liffey to be maintained as far as possible," a spokesman said.