Conolly's Folly is exactly the kind of odd landmark that makes the Maynooth and Celbridge area worth more than a quick university-town stop. The huge stone arch was built in the 1740s on the Castletown estate, partly as a display of status and partly as employment during hard times.
The monument stands in open ground, so the pleasure is in seeing its theatrical scale against the fields. Pair it with Castletown House if you want the full estate story, or use it as a short detour while exploring the quieter lanes west of Dublin.