It is a pity that Shanaghan hasn't received a reply - are conservationists so self-absorbed that they can't expain themselves?
I don't know enough about the genesis of the building to give an authorative evaluation, except to state that Archer's Garage seems a robust example of early modernism in Ireland, a corner design with large-scale spans accompanied by sufficient 'decoration' to relate it to the street-scene. However, the Corp. must have a report on why it was listed, and a bit of prodding in the right quarters should reveal the reasons for listing.
Why ARE the defenders of the Garage so silent? Shanaghan deserves a few more answers, eh!