Might be a bit late for this answer (which probably isn't accurate, but is how I understand it) but never mind, here goes:
Terminal 3 was developed in the 90s for Irish Ferries to berth their new vessel, the Isle of Innisfree, together with their own dedicated building on Salt Island. Accordingly it made sense to fit a passenger walkway directly to the ship. Stena however at that time was operating freight vessels (Stena Challenger, then Stena Forwarder) from Terminal 4. All foot passengers were carried at that time by Stena Explorer on her 3 round trips daily service. When Stena Adventurer arrived on the scene she was really intended to be a glorified freight vessel. Terminal 5 was developed only to allow a better draft and double tier loading, whilst foot passengers remained the mainstay of the fast-craft. Only when the Explorer's trips were cut back was it considered that foot traffic would have to be handled by Stena's conventional facility. The Stena buses which were originally intended to act as a 'park and ride' from the long stay car park near the fish dock were pressed into further service, commuting between the main terminal building directly to Terminal 5.
Think that's it, unless anybody knows different.