There are three types of bus transport to Dublin city:
Aircoach [9] express service (large blue bus) connects the airport and the city centre and many of Dublin's major hotels, most of which are on the south side of the city. Buses leave the airport every fifteen minutes and the journey time to the centre is approximately thirty minutes. The cost is ?7 single or ?12 return. Aircoach also offers services to other destinations within Ireland, including Cork and Belfast.

Beware of taxi drivers trying to pick up passengers at Aircoach bus stops. They are strictly forbidden from doing this so do not get into any taxi no matter what they say.
Dublin Bus [10] offers an express AirLink service (routes 747 and 748) every 10 minutes at peak times to the city centre and bus station for ?6 or ?10 return. Some of these services now use the Dublin Port Tunnel to avoid the city traffic and can reach the city centre in minutes.
Dublin bus also have a number of other local routes that serve the aiprort, and these offer substantially cheaper standard services to the centre and further afield in the southern suburbs: these are non-express and stop significantly more times going to and from the airport. Cost is ?2 and buses run every 10-25 min depending on time of day. You can save 10 cents by purchasing a Travel 90 ticket for ?1.90 in the ticket machines next to the airport bus stops, the ticket also allows you to transfer on to any other Dublin Bus services for up to 90 minutes, saving you another bus fare should you need to transfer.
The 16A goes right through the city, stops at O'Connell Street and continues up Georges Street and, finally, to southern areas of Dublin.
The 41 takes a slightly more direct route and finishes on Lower Abbey Street. It stops at O'Connell Street and close to Busáras (Dublin Bus Station).

Depending on traffic, journey times can vary from 25 minutes to over an hour. These buses are considerably cheaper than AirLink and Aircoach. Both of these local bus services stop across from Drumcondra train station which is on the Dublin-Maynooth commuter line. Some trains on this line continue past Maynooth and serve stations as far away as Longford. All Dublin Bus buses (except AirLink) do not give change and fares must be paid in coins. Ticket-machines near a few outdoor bus stops, including one at the airport, do not require exact change. Tickets can also be purchased at the newsagent inside the airport. Luggage racks are limited on the local buses, and it is not unknown for drivers to turn away travellers with packs that cannot be stored.

A taxi to the city centre should cost around ?20 to ?30: it can be comparable to/cheaper than the bus options if you are in a group of three or more (as well as a lot less hassle). Taxis are legally obliged to provide an electronic receipt detailing the fare, distance and other pertinent details. However, they often do not furnish such a receipt.

A metro connecting Dublin Airport to the city centre is planned, but no work has started on this yet.

Unless your destination is Dublin City, it is probably best to use one of the extensive range of other bus services that stop at Dublin Aiport and so avoid the city centre traffic.