John's Hall is a sober neoclassical building from 1833, first used as a Mechanics Institute and later as a public building. It now works as an exhibition venue, but the surprise is outside.
The Seffin Stone standing near the hall was moved from nearby Seffin. Its inscriptions are prehistoric, and local folklore grandly claims it marks the centre of Ireland and the seat of Finn McCool. That may be a stretch, but it is exactly the sort of odd local survival that makes Birr more than a castle visit.