Maynooth is a small town and everything worth seeing is within easy walking distance. From the railway station to Maynooth Castle takes about five minutes on foot; from the castle to the seminary campus and back to the canal towpath adds no more than another 20 minutes. You do not need a car or any other transport to cover the main attractions.
The canal towpath extends both east (toward Dublin) and west (into Kildare) and is popular with cyclists. Bike rental is available from a couple of shops in town. A half-day cycling the towpath east to Leixlip or west toward Kilcock is a good way to extend the visit without needing a car.
For day trips to Carton House demesne, Conolly's Folly, or the Kildare countryside, a car is useful unless you are happy to cycle country lanes. Taxis are available from ranks near the main street.