How big a gamble is booking 2 separate round trips?

I do this sort of thing quite frequently for various reasons. I typically build in an extra day in the initial arrival city. In Europe, that is generally one overnight at the initial arrival city, then move onward the next day; in Asia, it is generally 2 overnights (one full day) in the arrival city.

Would building in a huge layover be enough of a safety net?

I just experienced the "weather" cancellation of a British Airways connecting flight within Europe, delayed by one day our return to the US, something that had never happened to me before. I think what you are considering is a definite risk, one I would not be willing to take, but YMMV. What would it cost you if it happened?

Given that the flights you are looking at are still 5 or 6 months out, I would monitor the prices (highly recommend tracking prices on google flights). I have seen prices fluctuate wildly on flights I am considering for future trips. I just booked Delta One seats to Paris for mid December, at the lowest prices I have seen in months vs what I saw in August/Sept...I am getting used to booking flights much closer to travel dates than I ever did before.

This article describes our experience...we just returned from a trip to Europe, and checking bargain prices the other day, we could not resist going back over for XMAS...https://www.cnbc.com/2024/11/03/flights-to-europe-fall-to-3-year-low.html

Making sure I understand your plan. You want to go to Dublin in spring. The prices for direct flights to/from Dublin from Newark are more than direct flights to London. You'd like to take the less expensive option (London) and book a separate round trip flight between London and Dublin, unconnected to your USA flights. As long as you give yourself plenty of time on this second ticket you are probably going to be ok. You might want to book a slightly higher class of ticket for London-Dublin (one that allows changes) in case you do have a delayed flight situation. As well as researching other flight options and times so you know what your plan B options might be.

Have you tried an open jaw ticket from Newark to Dublin and then coming home from London to Newark (or vice versa)? That would only require a one way flight from Dublin to London (definitely do this the day prior to your USA return flight, do NOT fly on the same day).

You could plan to overnight in London, as suggested above, and continue to Dublin the next day ( BTW London City Airport ( LCY) is a very small, convenient airport if overnighting in central London and flying out the next day). The DLR ( Docklands Light Railway) travels from central London to London City Airport in about 35 minutes.

You also could buy travel insurance which would reimburse you for the cost of the London to Dublin flight if you missed it due to the U.S.-to-London flight being delayed or cancelled.

www.Skyscanner.com lists flight schedules and fares from London airports to Dublin.

My time, and not having to change terminals at Heathrow is worth quite a bit to me. If it were a two-segment trip anyway, I'd check air partners at my home airport. For example, I use United a lot, so I'd look at Lufthansa flights to Dublin via western Germany, if they exist. No more "roundabout" than your OP proposal, IHMO.

I avoid BA because it is so hard to book seat reservations, or has been in the past. I would also remind you that England is no longer a full member of the EU, which tends to make entry and exit slower. Don't know the actual situation for your scheme.

This was back before the "Celtic Tiger" economy, but because my (born in Pennsylvania) wife has a very Irish maiden name, one time we flew from seeing friends in Dublin, to London for the rest of our vacation, she was treated like a job-stealer trying to sneak into the UK on a tourist non-visa! She was questioned up, down and sidways. We had return tickets to show, by the way.

How much is a fully-refundable advance ticket for the flight to/from Ireland from LHR? Look at the price of a last-minute walk-up fare on the same itinerary. The difference vs the $125 would be the cost of the hedge if you had to change the flight.

The proposed layover sounds deadly - is there a flight a bit earlier? An overnight stay in London [or Windsor?] may degrade the momentum for your trip. How long do you intend to stay in Ireland?

I agree with checking walk up type price for the London to Belfast flight to see what you might be up against, if for some reason you miss your short hop flight. On Google flights I see this for tomorrow round trip $172 economy.

Personally I would take the risk with a $125 flight. By the time you add in two overnights in London, your savings decreases.

If you have AA or BA status, you could spend your long layover in the lounge for more comfort and free food.

Check prices of Aer Lingus from Heathrow to Dublin.

The reason.....United and Aer Lingus both operate from Terminal 2 at Heathrow. Therefore, no changing terminals. (British Airways operates from T5 for flights to Dublin.)

If you don't have checked luggage, and you have your boarding passes for the LHR-DUB flight, you can use flight connections and bypass immigartion. You will still have to go through security.

If you have checked luggage, leave time to go through immigration and collect your bags.

Do you have to fly out of Newark?

Have you checked JFK? Delta flies from ITH to JFK and then JFK to DUB.

Aer Lingus flies nonstop from both EWR and JFK to DUB.

Icelandair flies to Dublin from both JFK and EWR via KEF (Reykjavik.)

Since London or even England is not part of your trip, I would not want to deal with Heathrow. What about flying round-trip Aer Lingus?

Thanks everyone— this is helpful and l think I need to continue to explore other options. Im at that chaotic early planning stage where I’m still trying to coordinate an itinerary and dates and flights and pet care, etc.

Re your questions, we are probably spending just under 3 weeks in Ireland. Relatively late change of vacation plans because the Scotland trip we were planning together with friends is now on hold for another year or 2. We usually like to fly nonstop and are willing to drive 4-5 hours to a major airport to do so. It is a shorter and much less stressful drive to Newark than to JFK. We’ve flown out of Toronto in the past, as well.

In 2024 we drove to Newark and flew n/s on United into Rome and out of Milan. It worked well for us and splurging on business class on the outbound flight gave us access to the Polaris lounge at Newark prior to our flight. I’m not exclusively considering United but the nonstop flight availability out of Newark is good, and Polaris class plus a slightly more generous carry on size allowance are both pluses.

Flying direct to Dublin would definitely be easier and less stressful. I just wish the price difference wasn’t so great.

The return is more complicated because our trip ends in Belfast. Flying to the US from Belfast is absurdly expensive so we’ll either have to take the train to Dublin and fly out the next day or fly to London to fly out the next day. The open jaw Newark to Dublin and London to Newark is slightly less expensive than the Newark to Dublin round trip but I’m not sure it’s enough of a difference to try to add a one way flight from Belfast to Heathrow. Lots to consider!

If you're driving from Ithica, you might look at the Aer Lingus nonstop flights to Dublin from Hartford/BDL - just note that they don't run every day.

Up until our most recent trip we had gone 3 years of flights that all had significant delays or changes. For that reason I stress over connections on the same airline, let alone seperate tickets. So a month ago I was stressed until the time my flight left home on time for Barcelona with a 4 hour layover to catch a flight on another ticket to Lisbon. As it turns out I could have caught an earlier flight.

It's a gamble, so my question is how comfortable will you be, and what will the backup plan be?

Up until our most recent trip we had gone 3 years of flights that all had significant delays or changes. For that reason I stress over connections on the same airline, let alone seperate tickets.

I agree with Allan, the tweaks and changes to flight times and connections right up to departure day make me nervous. For me, doing what the OP first suggested, would introduce an unacceptable risk that I don't want to ever deal with. It sounds like the OP is going to look at her options for accomplishing this trip.

I avoid LHR whenever possible for connections - we recently got burned by this, my return flight from Paris was to connect in LHR for the US bound flight. Well, the night before our flight, British Airways sent me a cryptic email saying there had been "a change" to my flights, without directly saying my first flight had been canceled - I had to go to flightaware to learn that my flight had been canceled (BA later claimed it was due to the the weather). The only sign that something significant had changed was the BA deleted our boarding passes for CDG to LHR, yet BA looked forward to us boarding the LHR-DFW flight the next day! Fortunately, the booking was through American Airlines, and their customer service bailed me out, put us on the nonstop from CDG to DFW, the following day.

An extra day in Paris was not the worst thing in the world...for me, the only reason to fly to LHR is because London is my destination (it was the first destination of this trip, where I landed from the US). I will never again transit LHR on a trip to or from Europe. And I repeat that I will never again fly British Airways, their customer service is non-existent. Our upcoming trip to Paris at XMAS is on Delta, a nonstop from ATL.

The return is more complicated because our trip ends in Belfast.
Flying to the US from Belfast is absurdly expensive so we’ll either
have to take the train to Dublin and fly out the next day

That's really not a problem. There are frequent and cheap coaches between Belfast and Dublin airport.

In the very worst case that you have to buy brand new tickets to Dublin/Belfast, you are likely to still be ahead against your $2500 savings. And I like CLs suggestion to pay for flexible tickets if you want to insure against a delay arriving in London.

I think the OP is a very clever chap or lady. I guess the latter is more likely, based on the username. And probably irrespective of the username.

As Badger said, no need to take train from Belfast to Dublin and fly out next day. We took coach direct from Belfast to Dublin airport in the morning and flew home that afternoon. As you probably know, for flights from Dublin to USA you clear immigration in Dublin before boarding your flight. This is another reason to fly home direct from Dublin rather than connecting at LHR.