Heathrow to Ireland- when to arrive and when to leave London?

There are four of us, so probably the best time/cost value is a taxi or Uber.

That is the slowest and most expensive option. Jubillee Line from Waterloo to Green Park then Piccadilly Line to LHR.

For being at the airport, I would be there two hours early (you could even do 1.5 hours but I would to do two to be safe). In terms of getting there, I would not try to get a taxi (unless you have a lot of luggage or someone in your group has mobility challenges) as you risk being stuck in traffic since you're coming from very central London. I would either take the Jubilee line from Waterloo to Bond Street and then get the Elizabeth line (about 50 mins) or take the Bakerloo line from Waterloo to Paddington and then get the Heathrow Express (about 40 minutes, but more expensive). As a comparison, right now Google maps is saying the drive time is about one hour. All that is to say, you are fine to leave a little before noon so you should be able to fit in a morning activity if it's not too far from your accommodation.

Thanks for chiming in. I’m trying to see if I can squeeze in Westminster Abbey that morning from 9-11. I think it could work?

When looking at the price of the train to get to LHR, it would be $95-$167 for the four of us and would take 51 minutes, not including the time it takes to walk to the station. A taxi or uber at that time is estimated to be about an hour (according to Google maps set for that date and time), and costs roughly the same. I don’t see the big savings of the train. What am I missing?

Dang it. Looks like Westminster Abbey doesn’t open until 9:30 on Tuesday. So much for that.

Tube fare from Waterloo to LHR is £5.80. Elizabeth Line from Bond Street is £13.90.

Use contactless for either option.

TfL.gov.uk

If you book the Heathrow Express ahead of time you may get it for £10 each. Add on the tube fare to Paddington, and that is cheaper than the Elizabeth Line.
If you attend 7.30 Morning Prayer or 8am Eucharist at the Abbey you will at least see inside it, if not tour.

Your taxi estimate us optimistic, to say the least. I would estimate £95-100, pounds not USD.

Not a taxi. Reserve a car service such as justairports.com which I have used many times as have others on the forum. You can check online telling them from where at what time and how many people and how much luggage and you get a quote of price.

For the four of you, a pre-arranged car service might be the easiest and should cost less than a taxi. Not as cheap as the Underground or discount tickets on the Heathrow Express.

It picks you up from in front of your hotel and drops you at Heathrow. You will know the cost when you book it.

Of course, like a taxi, traffic may add time to your trip.

price of the train to get to LHR, it would be $95-$167 for the four of us

where did you get such expensive prices?

If you take the tube all the way it is £5.80 each, 4 adults is £23.20.

Elizabeth Line from Bond Street including the tube from Waterloo to Bond Street and then on to Heathrow is £13.90 so £55.60 for the 4 adults - these are way lower than what you quoted...

it isn't clear in your posts if the 4 of you is 4 adults or maybe 2+2 kiddoes.

In fact what everyone forgets to mention is that children under 15 travel free on Heathrow Express as long as there is at least one adult also travelling, no railcard needed. That applies even to the £10 discount fares.
They still need a ticket, but it is a zero value ticket.
And the adults can still get national railcard discounts - senior, two together, disabled etc.
So 2A and 2C could travel for £13.30

Hi again. Our group is two adults and two students, so there is a discount, but not as much as with children. The prices I was using for the tube/train were from using Rome 2 Rio, and then clicking on the ticketing option, which brought me to Omni.com (is this a reseller?). Rome 2 Rio had estimated a taxi at $100, but I had already reached out to Just Airports and they quoted me £100.

Yes they are a reseller.
The bargain Heathrow Express fares are only available on their own website. The upper end of the price range is probably for business (first) class which would be a total splurge for a 15 minute journey. Also tube fares bought as add ons to HEX fares are always more expensive than paying in the normal ways.

"Yes they are a reseller.
The bargain Heathrow Express fares are only available on their own website. "

Thank you!

Rome2Rio is probably trying to put you on the Heathrow Express, which is way more expensive unless you buy well in advance. For a journey where you'll be one operator the whole way (TfL in this case), Google Maps is probably the better way to search.

Rome 2 Rio can be an interesting way to see options, but their times, routes and prices can be absolutely miles off and sometimes comical.

Using them and then a reseller instead of the service provider can really lead you down the rabbit hole I'm afraid...