Dublin Airport, (Irish: Aerfort Bhaile Átha Cliath) (IATA: DUB), is an international airport serving Dublin, Ireland.
To quote from Joe O Murchadha 22 APR 2012 from Migration Policy in Ireland:
Dublin Airport is unique amongst the major European hub airports, insofar as it does not have an effective International Transit Zone, save the minor exception of the "200s" gates in T1. All passengers transiting via Dublin are required to pass Irish Immigration before proceeding to their onward flights, even when they have no intention of leaving the airport or visiting Ireland.
The Transit Experience
In effect, the experience of transiting in Dublin differs greatly depending on (1) which gate your arriving and departing flights happen to use, (2) whether or not you have your onward boarding pass, (3) whether or not your luggage is checked to your final destination and (4) what passport you hold.
Passengers arriving at the "100s" gates (familiar to most Ryanair passengers) walk under the departure area and follow the elevated glass corridor before arriving at Immigration. Then they must walk the full length of the T1 baggage hall before climbing an escalator and taking two sharp bends before being spit out awkwardly at the main T1 security search point.
Passengers arriving at the "200s" gates (AirFrance/CityJet, AerArann/AerLingus Regional, FlyBe and others) follow a similar, if shorter route, unless they happen to have a connection departing from the same area. If they arrive and depart from the "200s," they can board their next flight without passing any security or Immigration checks at all. Yes, these are the gates normally used for flights to Donegal, and yes passengers arriving from abroad can avoid immigration entirely and enter Ireland unchecked this way.
Passengers arriving at the "300s" gates (Lufthansa, Turkish Airlines, BMI and others) meet Irish Immigration in the recently revamped old US PreClearance area before arriving in the middle of the T1 baggage hall. From there they follow the same route via the esculator at baggage belt 1. All gates in T1 and T2 can be accessed from here.
T2 arriving passengers have a slightly easier time with a dedicated transit corridor equipped with its own Immigration counter and security check. Passengers can then easily access the T2 gates or alternatively can follow the warren of corridors to gates in T1.
Thankfully, airside transit between T1 and T2 does exist in Dublin, however unlike landside, only gate numbers are signposted and it is assumed that passengers know all the "100s, 200s and 300s" gates are in T1 and that the "400s" are in T2. Another difficulty is the sheer distance and the lack of airside transit desks for most airlines. A passenger arriving into T2 unfortunate enough to have a connection from the "100s" gates in T1 had better have their hiking boots on and, in most cases, had better have their boarding pass in hand as they will not find a transit desk en-route. Non-EU/EEA Ryanair passengers cannot transit by following the signs even if they do have their onward boarding pass as they require a Ryanair “Visa-Checked” stamp on their boarding pass which is only available in the T1 Check-In area. Unlucky Non-EU/EEA passengers normally learn this at the boarding gate when it's too late to go back and they miss their flight without refund.
Currently there are daily flights to New York (AerLingus and Delta) Newark (United/Continental) Boston (AerLingus), Chicago (AerLingus and American Airlines), Philadelphia (US Airways), Atlanta (Delta), and Orlando (AerLingus).