Actually, No! Irish ferries Terminal (Known as Terminal 3), including the walkway, which is only accessible from the Irish Ferries Building, was constructed several years before Terminal 5. It was first used for the Isle of Innisfree, then Isle of Inishmore prior to the introduction of Ulysses. At that time, Stena were using the (now defunct) single tier loading Terminal 4 for ro-ro only, as all pax travelled on Stena Explorer from Terminal 1.
Terminal 5 was later constructed for the introduction of Stena Adventurer and also involved land reclamation to make sufficient space for their trailer park.