Transferring on international flights

You should not have to go through customs as long as you're not leaving the airport, your bags are checked through to your final destination and you have a boarding pass for your next flight,

However, there's a good chance you will have to go through security again. Plan for that.

If you fly to UK via Ireland, because of the Common Travel Area you will pass through immigration in (I assume Dublin). Your onward flight to the UK is considered domestic.

As to where you pass through customs will depend on how you are ticketed, if you are on a single PNR you will not see your bags in Ireland and you pass through customs at your destination airport in the UK, which will take all of two seconds. If you you miss your onward connection the airline will put on the next available flight with capacity FoC.

If you are self connecting on two seperate tickets, its the same MO as above for immigration, however you will collect your bags and pass through custom there. Then you will have to re-checkin/bag drop again. One hour 40 is probably not enough for this as checking is likely to close at least 45 mins before the timed departure. If you miss you onward flight you will have to buy one at the walk up price. Or get a ferry/train.

I do think that will be enough time. Check for your connection time for the return as I know you go through US customs in Ireland before boarding a flight back to the states (assuming you are flying from the US). If you booked it all on one ticket, there is a minimum transfer time airlines must give you and that amount of time must be sufficient to transfer to your next flight. If it isn’t enough time, that will be on the airline and they will get you on the next flight.

"Check for your connection time for the return as I know you go through US customs in Ireland"

Immigration, not customs and isn't it an optional pre-clearance as well for faster processing the other end?

If the OP is on single PNR I would say they are absolutely fine as they have protected connection. If its a self connect, I would say its a very risky connection, if the inbound flight half a hour late or there is a congestion/queue at immigration the bags slow coming off the aeroplane, that onward flight is seriously at large with all the consequences that follow.

If you are on one ticket, you should be fine as you shouldn’t have a need to go through passport control (not customs). If you are on two separate tickets, you just took a gamble and I hope you are lucky.

Wow, you have you been given some good information and some bad information.

First you have to realize that immigration (passport control) and customs are two different things. Immigration is for you and your passport. Customs is for your stuff.

When you arrive in Dublin, you will go through immigration. Flights from Ireland to the UK are in the Common Travel Area and there will be no immigration in the UK. Your bags should be checked through to England. (If this is on two tickets, the only difference is you will have to retrieve your bags and recheck them.) When you arrive in England you just go.

On your return, if through Dublin, and you are going to the U.S. you will go through U.S. immigration and customs preclearance in Dublin. This is not optional. When you arrive in the U.S., it's like a domestic fight. No immigration or customs. If Canada, you will transfer at Dublin and go through immigration and customs in Canada.

This website should help:

https://www.dublinairport.com/flight-information/connections/connection-guide

"If you are on one ticket, you should be fine as you shouldn’t have a need to go through passport control (not customs)."

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Not true. Irrespective of ones ticketing arrangement because one is entering the CTA you pass through immigration in Ireland.