If the building is recent or currently a proposal, planning files, including drawings, are available for inspection free of charge at the public counter in the local authority office. In the case of Dublin city, go to the Civic offices at Wood Quay (not open at lunchtime). Older files (we’re talking years, not decades) can be ordered up from the archives, but it is a costly business. Older than that, your only hope is that the building was a prominant one: the plan may have been published (for example, in Irish Architect, awards catalogues, etc) or, in very few cases, the original drawings may be lodged in the Irish Architectural Archive, located at 73 Merrion Square.