Having been in Dublin a couple of times since living in this country, I have to admit that I understand why people complain so much about the bus services. Price is not good, you have to have the exact change, no good deals on weekly/monthly passs, etc etc.
This is what I was used to before coming to leave here...
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People are considered "young" if younger than 21 (big arguments here, as most people will be in college until they are 23 or later)
For turist, you also get this:
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(this is fairly new, haven't heard about it before, but it sounds like a good deal)
And if not sure, it's always good to get a "metrobus" instead of the single ticket, you save a lot of money, and it can be used by more than one person...
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Isnt' it great that you can get to the airport using just one ticket?