Euros and pounds

You could probably get away with £0 in the UK at least. I haven’t handled any cash in four years.

But yes ATMs are available.

For the UK, highly likely you won't need any cash at all. Your plan of using an ATM if required is a good one.

I spent over two weeks in Scotland last June and 12 days in London last September. Did not use cash at all, paid for everything with credit card. Yes, ATMs are available and not hard to find.

Plenty of ATMs in Dublin and Ireland. And like the UK, you may not even need Euros in Ireland. The payment technology in Europe exceeds that of the US.

While high street banks across the UK have been closing more branches in recent years, cash machines are still widely available- and more importantly, as others here have stated, there are very few situations here where you would need cash. You may want to withdraw a small amount when you arrive just in case you need it, but I pretty much never carry cash myself. The only instances that would require cash I can think of would be for tipping where you can't leave it on the card (e.g. in a hotel room for the cleaning staff).

If you’re only going to Northern Ireland, isn’t their currency pounds?

I'd imagine you could survive pretty easily everywhere you're visiting cash-free, except that in Ireland you'll need euros to ride on the buses. Full disclosure: I've never been in Northern Ireland. ATMs are ubiquitous.

In the past, most banks required advanced notification of foreign ATM use. My bank doesn't require notifications anymore. I suspect most banks don't require this now, but, to be safe, I would check with your bank.

The AAA is still recommending this notification.
https://cluballiance.aaa.com/the-extra-mile/advice/travel/avoid-when-using-atms-abroad

The tips on this site don't mention the notification: https://www.ricksteves.com/travel-tips/money

I agree with the others and will ask if you use ApplePay or GooglePay at home? Honestly, my last few trips I have ApplePay'd my way across Netherlands, France, England, Orkney and Shetland.

I do have some cash on hand, mostly left over from prior trips. Last trip I did get some Euro as I was doing restaurants with others and it was good to have cash to exchange in case the restaurant would not split the bill or if someone wanted to use their CC.

In England even a number of churches have installed contactless stations for you to give them a donation for a visit!

Depending on how many days you will be gone, I'd say you would not need more than 50 GBP and assuming fewer days in Ireland, maybe 25 Euro. That will still give you some to come home with for your next trip.

Ruth, OP stated Ireland and Northern Ireland, and mentioned Dublin also.

Thank you all! Confirmed my thinking.
Betsy

Technology is great when it works, but from time to time there are glitches. We've experienced ATMs that malfunctioned, card readers that did not work, and a few places that only wanted cash. Since most of your time will be in the area that uses pounds, you might want to get 100 pounds from your local bank just in case the little green men in the computers decide to misbehave. And you can probably risk card only in Ireland.

yesterday I was sure I would need cash. I went to a teeny tiny shoestring museum only open 6 days a year (Amersham Fair Organ Museum out in the boonies of single track lanes - really niche - loved it, Thanks Ian Visits newsletter) which doesn't charge admission and makes what small income it gets with a raffle and selling inexpensive teas and biscuits.

I had forgotten to put what few coins I have in my pocket (sometimes I think I will need them, most of the time they sit in the coin tray on my desk and gather dust) and when I got to the tea counter I ordered a cup of tea for my wife, 2 biscuits and a strip of raffle tickets. I then realised it was probably cash only and began to dig for my wallet to get my emergency £10 note, saying that it would be the first time I'd paid cash in 4 years. Lad behind the counter said never mind and reached behind to get his contactless machine. Waved my phone near it, job done. No fuss no muss,

Fabulous museum if you like Belgian and French fair organs playing at full whack. Said it was niche.

https://afom.org.uk/

An example of what TC mentioned. In 2022 at Rosslyn Chapel its card processing system went offline so cash was needed. Fortunately, we had some. This past December in London the only cash I used was a tip at a small tea place. I didn’t need to use cash, but wanted to ensure the two individuals that served us each got their share.

The assumption that you should have cash for when the card readers don’t work assumes that the merchant is even going to be set up to accept cash. And I’m seeing more and more merchants in Europe that aren’t even set up to accept cash.

I’ve seen it here too, if the credit card machine is down, the business just closes. I saw it a few weeks ago at a bakery near my house.

You might need a modest amount of cash. In the UK, some public toilets require coins.