At the time the airport was designed Aer Lingus was by far the largest airline and still had a lot of influence. Ryanair was a small, regional airline with a few 737-200s and grandiose plans, but nothing else. Aer Lingus wanted air bridges, which forced Aer Rianta into a new terminal rather than an extension and refurbishment of the existing terminal. Between the sod being turned and the airport opening a lot changed, so some revisions were made to accommodate airlines wanting to use steps, nit air bridges as they previously wanted. As I understand it the airport was designed to deliver exactly what airlines initially asked for. 4 air bridge gates and 4 gates for non-airbridge users, to accommodate mainline, low-cost and regional operations.
As I have said, the building is not a problem, the airlines using it are.