Dublin Bus 16

The number 16 bus will take you from Dublin airport all the way out to Rathfarnham. It takes in a lot of neighbourhoods along the way so can be a bit of a tour bus around Dublin but it is a lot cheaper than a taxi or the airbus.
The number 16 is always busy though, full of students, travellers from different countries and often stuffed to the roof with luggage.
A negative is that they have a sign at the airport to pay with exact change and as a Dubliner that is a known fact but i think for visitors it could be made a little easier by having ticket machines and other ways to pay other than having to arrive in Dublin carrying little bits of coin.

As I read in another review if they think the Air-​coach is the cheapest and easiest way to the airport they are mistaken. The public transportation is just as convenient and it is cheaper. The public buses takes you to the same location as the air+coach and it is only 1.85 Euros compared to the 7 Euros for the Air-​coach. The time might be about 5 minutes longer than the air-​coach but that is still not worth it when you are paying 5.15 Euros more for a ticket.
The drivers are always helpful to tell you when you are nearing the airport and which of the stops you need to get off to at depending on what type of flight you are taking. So it is as convenient as any other type of transportation. And I am a sucker for double-​decker buses. I always love sitting in the front the the top deck of the bus.

Trip to the airport? Cab schmab.

Anyone who said the Aircoach is the cheapest way into the city is mistaken! Quickest or most convenient, well, they still have that point on their side.
Dave’s said most of what I would have. I’d add that travel time depends hugely on traffic — I’ve had rides between the same places take anything from 30 minutes to an hour and 20. The price is right and it’s very convenient to city centre and a good number of neighbourhoods along the way and beyond, too. Just plan for a very long trip at times of heavy road traffic and/​or bus use. It’s not just an airport bus, after all, and every stop takes time to complete.