Would this be complicated to do at Heathrow?

First, when you priced the tickets did you use the multi-city option rather than two one-ways? For example, the first leg would be LAX to DUB and the second leg would be LHR to LAX.

Ryanair doesn’t fly from Heathrow. You will have to switch airports. You will have to go through immigration and customs and then travel to Gatwick or Luton or Stansted. I wouldn’t want to do this unless I were planning to spend my arrival night in London.

Yes, it would be doable.

No, there wouldn't be any complications with UK Border Control.

RyanAir doesn't fly from Heathrow, so you would have to travel to another airport.

If there are any delays, you are on your own and would have to buy new tickets if you miss your flights.

You are looking at multi-city tickets into Dublin and out of Heathrow, and not two one-way tickets?

First, make sure you're looking at a multi-city booking and not two separate one-ways. I just checked for Sunday May 24 through Sunday June 7 and found BA tickets (LAX-DUB, LHR-LAX) for $1158 vs $1236 RT to LAX-LHR.

Yes you can self-connect at Heathrow, but you would likely need to clear entry immigration, pick up your bags, change terminals, and go back through security. Make sure you double check that the Ryan Air ticket is from LHR, too, as the ultra low cost carriers love to fly into smaller airports. You should also pay for your carry ons and checked bags in advance; they charge an arm and a leg if you wait until check-in. I would give yourself at least 5 or 6 hours to make the connection, since neither BA nor Ryan Air will care if you miss the connection.

I would price the multi-city trip as above, and then check the price of return flights from London to Dublin and back, on one of the airlines that flies out of LHR, probably BA or Air Lingus. In fact, price LA-LHR-DUB-LHR-LA all on BA as one ticket and see which is cheapest, that way BA will be responsible for getting you to your destinations if anything goes wrong.
BTW, The cheap carriers are never as cheap as they look, there’s generally a charge to check luggage and much tighter limits on carry-on bags than the standard airlines

What you really don’t want to have to do after such a long flight is to change airports in London - a real hassle.

Have you already ruled out a multicity ticket into Dublin and out of London?. This is often cheaper than 2 one way tickets. It's not intuitively obvious on the BA website, but you can Google how to do it

There are risks associated with using a second , separate ticket on a same day flight. Things may go according to schedule, and with sufficient time between flights, there would be no problem. However, if your first flight is delayed or cancelled, then your second flight would be missed and Ryan Air would have no obligation to get you on the next flight. You would need to buy a new ticket at walk up prices.

AFAIK, Ryan Air doesn't fly out of Heathrow. So you need an ETA to enter the UK, collect your luggage, travel to either Stansted, Luton, or Gatwick to check in for your Ryan Air flight to Dublin. Which will take several hours. And of course, Ryan Air charges for every little thing, including checked and carry on bags. If you go that route, you might consider spending that one night in London and flying to Dublin the next morning.

We just about always fly a major carrier to Europe and from there book with Ryanair or EasyJet on the next leg of our trip. I don't see how they can have such low airfares.

Only thing is that you may have to haul your luggage around the terminal a little and you may have to sit a couple to a few hours to get the departure times lined up.

And don't forget that if you step foot in England, you will have to pay for a ETA--mini-Visa of sorts--at entry.

As others have mentioned, RyanAir does not fly out of Heathrow, but does fly out of Gatwick, Luton, and Stansted (to Dublin). If you are going ahead with this plan I would definitely suggest you select a flight out of Gatwick as that's the easiest of the three airports to get to from Heathrow- there is even a direct bus, and it takes a little over an hour. Personally I would be a little nervous doing this as if your flight from LAX is delayed and you miss the Dublin flight then you will just have to book new flights at your own expense. I would build in lots of extra time to make the connection. It's definitely doable but a little risky.

Both British Airways and Aer Lingus fly between Heathrow and Dublin. When you look at their airfares and compare to the discount airlines make sure you price out everything you need with the discounters (Not just the advertised fare) as well as the cost of getting from one airport to another. With Ryanair all you get is the seat. You can take a small personal item on board. Do you want to take a normal size carry on bag? You pay extra. Do you want to check luggage? You pay extra. Do you want to choose a seat before check in? You pay extra. Do you want a bottle of water on the flight? You pay extra.

Sometimes the bigger carriers with less options cost less in the long run than the discounters--especially when you include the cost of airport transfers.

One other thing to remember.....when you arrive in Dublin you will go through immigration. However, when you travel from Ireland to the UK there is no immigration check.

Wow!!! Thank you to everyone!!! I should have mentioned the $1,000 difference was flying Premium Economy and I did look at "Multi-City" I did NOT know Ryan Air did not fly out of Heathrow and will likely just book with B.A from L.A. to Dublin with a connecting flight at Heathrow.

I really appreciate this forum!

Use Google flights to check what are the options for flights between 2 places. Also book direct with the airline but googleflights shows the airlines and the airport used which is essential when going someplace with multiple airports. Have you considered using Manchester instead of London?

I am doing some similar for my trip this fall. Arriving at Heathrow early. Around 7am. I always allow plenty of time when I have “unprotected connections “. So I will probably book an early afternoon aerlingus flight. Their plus fares allow you to standby for earlier flights on a space available basis. I’m fortunate to have lounge access so I have someplace comfortable to wait it out if I can’t depart on an earlier flight.