(Mandela House) House 6, Trinity college.

Mandela house/house 6 is located to the left off the main entrance jut after you walk through it, characterised by a large black door that is usually open during college term hours. Climb two flights of stairs (present at the entrance to the building by the door) and turn right, walk around the stairwell, and there will be a corridor that turns left. The bathroom is located to the right of where this corridor begins.

It is usually open during the daytime, and during college term. During college term, it may be open later into the evening, but generally remains closed during the summer as far as i know, you may have better luck than I did. It is a college bathroom, so no need to pay.