Clearing US immigration in Dublin

Hi, can anyone help with this query. I have friends going to Boston next week from Dublin. Do they clear US immigration in Dublin? If so, is it the case that once you go through US immigration you can't get back out again to shop, have a meal etc in the airport? So should all this be done before? Also how long does immigration take and what does it entail?
Thanks.

Presumably they have already completed the online travel authorisation

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When we travelled last year it took around 45 minutes to get through, there were long queues but plenty of staff on, the acutal questioning/finger print and photograph taking took about 15 minutes for 3 of us.

Once you go through you cannot go back out so make sure you have done all your shopping and eating before hand. The waiting area is not particularly large or comfortable and there were only a couple of coffee shops.

On the plus side when we landed at JFK it took less than 20 minutes to get our bags and go through passport control.

Not all flights go through immigration before flying to US. In that case you will clear in the destination airport

Only Aer Lingus fly DUB-BOS direct at the mo and this flight does clear US immigration in Dublin. If you friends wish to eat and shop at the airport then they should aim to be at the airport at least 3 hours before the flight.

US immigration generally opens 90 mins before flight time and queues could be an hour, or could be 10 mins, hard to predict, but it is advisable to go to immigration at the time the check in agent tells you.

After immigration there's a tiny coffee shop and some vending machines, no other shops, if they want food and to shop, they need to do that before they join the queues for US immigration.

JP1234 said:

On the plus side when we landed at JFK it took less than 20 minutes to get our bags and go through passport control.

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With the new facilities due in Terminal 2, not only will you clear immigration before departure but also customs so once you arrive you just collect your bags and go.