Is 3 hours enough time to transfer to a separately ticketed international flight at Heathrow?

You are cutting it to close. You need at least 4 hours or maybe 4.5 at least. If your first plane is late, as mine was this summer, your 3 hour window becomes 1hr. There are two immigration lines, not to mention security.

Here's something else to consider: Make sure you know exactly which terminal your departure flight will be leaving from. I was misdirected by the airport staff, got to wrong terminal, had to reverse, wait for the bus, get back on the bus, run like heck. They were boarding when I got to the gate. Have a great trip!

I would not fly in for my longhaul flight on a separate ticket the same day. Waaaaay too expensive to buy a new ticket if something goes wrong.

Yeah that is a little risky. Go to Ireland first seems less risky

Let me make sure I have this right....You are flying from Ireland to London and then transferring to a flight to the U.S.?

If so....no worries.....no immigration in the UK from Ireland.

Go to Heathrow Flight Connections and plug in your flight info. It will give you step by step instructions on what to do. This is especially important if you have to change terminals.

Why don't you leave Dublin the evening before and stay in a hotel at Heathrow Airport (Sofitel Hotel is wonderful and it has a skywalk into terminal 5 and easy access to other terminals). This way you can have a good nights sleep without the worry of making that connection in the morning for your transatlantic flight. I fly into Heathrow frequently and it always takes a long time to get through it these days. On both my trips this year, I have had delayed and cancelled flights, an IT computer crash, and lost luggage to deal with. Don't add stress to your vacation. Make sure you have a BIG connection time otherwise it could be costly. Alternatively, you could put Dublin on the front end of your trip. If you happened to miss this connection from Heathrow then it would be fairly easy to get another flight at a reasonable price. Also hand luggage is the way to travel.

The minimum check in times for flights to the USA is 3 hours, which doesn't allow you any time for your plane to taxi in, to get off the plane and transfer gates. You need an absolute minimum of 4 hours plus and wriggle room for flight delays.

You need to be at Heathrow for 9am. I wouldn't be happy staying in another country the night before such an early check in time.

It would be helpful to have some more specifics as to the airlines here as say both flights being in terminal 5 (or for that matter 2) is a whole lot easier than the other possibilities. I also presume that you would actually have checked in online before and would only be looking at a bag drop which potentially has a much closer-in time. Day of the week and time of the year would also be useful.

Check with your airline in Ireland - they may have a baggage agreement with the airline you are flying out of LHR and may check your bag all the way to your final destination!

Just make sure you have ALL your flight info before you check in. I work for Delta and we do this with our partner airlines!!!

Janet

Check with your airline in Ireland - they may have a baggage agreement with the airline you are flying out of LHR and may check your bag all the way to your final destination!

Neither BA or Aer Lingus will do this on separate tickets.

Janet may do this, but as a Delta passenger I can assure you that a lot of Delta agents will not ticket luggage through to the final destination if you are on 2 tickets

Heck I could not even get AirFrance to tag my bag all the way home on one ticket. Luckily since you have to claim and "recheck" at your first US city it was not an issue.