It's a very difficult and strenuous climb down rocks to get here so be prepared with good athletic shoes and a bottle of water. The lake itself is really impressive. Take a rest, enjoy it, and then prepare yourself to climb back up again.
Tricky to find and the cove next door had better swimming. Green lake was very green - looking pretty stagnant in fact. Maybe it's the time of year - end of summer, low rainfall, calm seas with no waves breaking into pool. Followed cairns along path until they ran out, then tried to find a way down. Very steep. Realised there was a way in from the north around cliffs. After passing large cove, stay low, don't head up the hill (where there are more cairns, confusingly). You'll find a series of cairns and blue dots painted on the rocks to show the way. It's rock-hopping, not an easy path. Pass caves, pass cliffs and descend to the green lake. Once you've seen it, return to the cove for a refreshing swim (there's a way down to the water on the north side). On the walk, rocks are jagged - wear long pants and even gloves if you have some.
Getting there requires shoes, water and it takes approximately 30 minutes walking from Limnionas. There is some rock climbing and rough edges, but it's worth it and even the less athletic of the group made it. The 'pool' water is nice and clear, the sea waves crashing a few meters away..