You're talking a 25/30 minute drive to the City from both places at least. That's not with traffic!
Both places are dead in the winter. During the summer, the beach in Myrtleville is always jam packed with locals especially teenagers from the surrounding areas (Crosshaven, Carrigaline etc). AFAIK bunnyconnellans has closed down so there's just the one pub there at the moment and one shop. I dont know much about Fountainstown only that there's a stoney beach and it's real quiet in the winter!
I'm sure someone else on here will be able to answer your questions in more detail!