If you're looking to visit this place, don't rely on Google Maps to get you there. It will take you to the opposite side of the River Corrib. If you did find yourself on the NUIG side, you can take the Quincentennial Bridge to the other side, and follow the road up over the hill. When you get the the stop sign, turn left, and stay to the left. You'll come to a newer castle structure on the left side of the road. Pass that, and continue to the locked gate. Everyone seems to ignore that obstacle. About 100 meters down the road is where you'll find Merlin Castle.