DART (Dublin Area Rapid Transport) is the train going through Dublin to the villages and towns (suburbs) north and south of Dublin. It is very convenient to use if your doing a daytrip to visit the places along the coast.
Tickets can be bought from machines or the booking-office . At Connolly Station the line for buying the ticket was very long early in the morning, but it was also very quick.
I took the DART to Killiney and back from Dalkey. A return ticket to Killiney/Dalkey is 4.10 Euro (February 2007). The trains are quite frequent and runs every 10 - 20 minutes and it takes about 25 minutes.
Usually the best way to get to Dalkey from Dublin is by the DART commuter train. It takes about 30 minutes and will cost about 2 euro. You can also catch the 7D bus from Dublin but it takes at least an hour so unless you actually enjoy bus journeys . . .
Take a DART (Dublin Area Rapid Transit) from Dublin city centre south. After half an hour you will reach Dalkey. (You might have to change trains in Dun Laoghaire though). Walk through Dalkey town to the seafront and start walking south towards Killiney.