Good morning friends, fellow travelers and experts!

My husband decided that he'd like to visit Ireland for his first trip outside of the US. I haven't been out of the country in 4 years and have never been, so I am really excited to plan this trip for us!

Quick question and I am hoping you could answer me:

I am flying out of Dulles and it looks like we'll probably connect in LHR before arriving in Dublin. Would we have to go through customs in both England and Ireland if we took this flight route?

I'd like to use miles to upgrade and this is the flight pattern that is eligible for the upgrade which is why I ask.

Thanks for your help!!

The UK and Ireland are in a customs union (I think), so you will just go through customs once.

You sort of go through Immigration twice eastbound, but - if your bags are through-checked - only go through Customs once.

If you're connecting at Heathrow to a flight anywhere in the British Isles, you go through UK immigration at LHR. If your bags are through-checked to another destination, those bags are, in theory, subject to Customs inspection at the final airport - and you won't see them being scanned behind the scenes. You then go through Security. On arrival in the Irish Republic, you're subject to Irish immigration, though for people arriving from the UK who don't need visas this is usually (but not always) highly perfunctory. If your bags aren't through-checked, you have to collect them in London, go through Customs (the work of less than a nanosecond in Britain) and check them in again landside

Returning to the US, most direct flights put you through US Customs and Immigration in Ireland. If you're connecting at Heathrow, though, there's usually no visible UK immigration (you pass a police checkpoint that's manned only when the IRA is up to its tricks again): you go through Security to the US departure gate.

PS: The UK & Ireland are, of course, in a Customs Union. But so are all the EU 's 27 member nations - with each other, with Turkey and with EFTA.

The UK & Ireland are in a Common Travel Area with each other. That means there are limited controls on movements - but because UK and Irish visa rules are different, the Irish require arrivals at airports from outside to demonstrate their eligibility to be admitted. For the Irish and Britons, this simply means a driving licence: for most other Europeans an identity card, but for everyone else a passport.

Thank you for all of replies. I would assume that my bags would be checked through to my final destination. We're not planning on stopping in London to sightsee or anything - only using Heathrow as a connection airport.

In looking at flights, I am unsure if I need to factor in the lines at customs into my layover to ensure enough time to get from one flight to another. Does that make sense?

The airline will tell you what the minimum connecting time is.

There are no queues at Customs in Europe. There may well be substantial (>30 mins) queues for UK immigration

If you're doing this on one ticket, airlines will factor in the MCT for an International to Ireland transfer in the ticket, and will rebook you onto the next flight to Ireland (of which there are loads) if things go wrong. If you're cobbling this together yourself, or using a third-party booking engine across two airlines, you're holding the baby (and possible cost) if you miss the connection.

Go here for average times:
http://www.heathrowairport.com/porta...t+Connections/

I'd add at least an hour if two separate tix are involved, and more still if you're not sure your bags will be through checked.