One of the cheapest way of commuting in Dublin is definitely "Luas". Its is the breed between a train and a tram (Like the ones till used on the roads of Calcutta). 90% of the people in Dublin rely on Luas. I got a temporary card for Luvas (which has to be recharged frequently) for 10 ten days, rather than getting a card for each time I board.
"Scary fact: The fine charged is immensely high if we fail to get a Luvas card before we board. Not only for Luvas, it could be anything done illegal, the punishments are severe and the officers are so sincere in that"