Besides having trouble with singular tickets, we can recommend getting a “Leap” Visitor Card which is a 1/3/7 days pass with unlimited use.
We also recommend NOT to buy it online (not an e-ticket/printable) but get it at the airport already (see above link) or at any “Circle-K”.
Route info / journey planner / realtime track:
- TFI.
- DublinBus.
Use any lane reaching stop #764 when coming from the north (city centre, airport) or stop #768 when from the south (e.g. speaker hotel). Lanes are operated there at roughly a every 20 minutes frequency and are at least: 145, 155, 46A.
Disembark at #764 “Merrion, Belfield Slip Road” and follow the ramp in driving direction; turn right at traffic lights. Cross the bridge and turn right in front of the fence/shrubbery (dont bring shrubbery) and follow instructions “from south” next.
Disembark at #768 “UCD Belfield”, cross the recessed sidewalk and two pedestrian crossings
into the parking lot. Follow the orange signage “UCD O’Reilly Hall”.
The entrance is here 
.