For safe diving, Sandycove Harbour leading to Scotsmans Bay, has to be one of the best spots on the East coast. You can snorkel out from the harbour and descend just outside into about 3m depending on the tide. There may not be a lot to see but if you take your time and head in a NNE direction there are some nice boulders and clean sand with small fish and crabs to spot. Watch out for the Cuckoo Wrasse who build nests with seaweed in late spring time. The area is almost free of currents but can give a bit of reverse pull on an incoming tide. Don't head out too far or you will enter the main tide stream!