Credit cards

Since you are getting euro, does that mean you will be traveling to the Republic of Ireland? If you go into Northern Ireland, they use the G. British Pound. That is likely way more cash than you would use. Many, would recommend waiting until you get to Europe to get your cash. For travel to Europe, I always consider Visa to be my primary card, however, I always bring 3 types of cards-AMEX, Visa, and Mastercard. Every once and a while we get to a country where AMEX is more widely accepted. Portugal and Australia were a couple.

One caveat on the cash. If you are staying at B & Bs, many Irish B & Bs require cash.

If you receive responses from those who've been to Ireland that's of course the advice to follow- but meantime. after widespread travels in many European countries it's been rarely accepted. I take mine for the Delta points and sometimes larger hotels or more costly stores have accepted it, but then it's always a nice surprise.
Sidebar- for years I used Capital One as my alternative to AmEx, but now they are on the Discover network. Great advice (as usual!) from here on the Forum was a Schwab card, fast and easy online despite that I have my credit freeze activated.
Edit: I agree about the cash but would get 1/2 and 1/2 currencies depending on the length of time for each

As noted above, you will likely find few places that want cash only. Credit cards, by card or on your phone, is now the norm. Visa and Mastercard are by far the most accepted. Amex is usually limited to hotels and higher end stores. Discover is not commonly accepted.

Spent two weeks in Ireland last month and Visa or Mastercard was widely accepted. Hotels were the only ones to accept my American Express. I took out 200 euros at a Dublin ATM and that was all I used the entire trip, mainly for tips. For the first time ever, I loaded a Visa card to my Google Wallet on my cell phone and used it for "tap and go" for the majority of my expenses. It was the preferred method of transactions.

We're heading to Ireland in a few weeks so I have no recent experience in Ireland, but we just got back from France. My sister has her amex as her first card on her apple wallet, it was rejected several times but not always. Once or twice she was able to use the physical amex card when the phone wallet didn't work.
We used a visa (bank of america) and Capital One Venture x, always physical cards, never on the phone. Both were accepted everywhere we tried although once or twice the tap didn't work and I had to insert it. That may have been operator error, you need to keep the card still til you hear the beep or see the notice, and maybe I rushed. Also the hand held machines aren't all alike, some you need to tap at the top and others flat on the screen.
NOTE all above is talking about credit cards.
For ATM/Debit cards, I only use those for getting money out of an ATM. For a few years I used my Capital one atm card with no problems, never a charge. Bank of America ATM cards are a bit more restrictive, for each country you have to find which is their partner bank (there's a list online). After Capital One bought out the Discover network, I got a fidelity ATM card (similar to Schwab, not charges anywhere). I carried both but only used the Fidelity ATM card for this last trip, not wanting to 'test' the Capital One ATM card on the new network.
But, just to reiterate, for credit cards I haven't had any problems using the Capital one, even after the Discover buy out.

I agree with everyone here. We just got back from Ireland (Republic of, and Northern). A couple with us kept trying to use their AMEX, nobody would take it anywhere. But as was said, maybe most big hotel desks would.

Also a popular topic here is that the Discover network cards are only accepted at a very few places. Using a Discover website to find the one ATM or the two restaurants in town that take it, isn't good enough.

We used a Citibank M/C everywhere in Ireland, no problems ever. In fact we loaded our Citibank M/C into Google Wallet, and I never had to take out the physical card. It was much faster to just tap and pay with my phone. And one restaurant out of about 40 asked for a tip. Nobody else mentioned it.

And I agree that 100 Euros or Pounds cash is plenty for most folks. Even many vending machines took tap and pay.

Regarding CapitalOne credit cards, I would think that if you have a CapitalOne Mastercard it will not be sent to the Discover network. For that to happen CapOne would have to end the card and replace it with Discover Card. This is different from how their bank cards were handled.

To reiterate Larry's point - my Capital one Venture X credit card clearly says VISA on the back. This card was accepted everywhere I used it in France last week and has been accepted for online payments for my upcoming Ireland trip.
The issue is with Capital one ATM/Debit cards - these were reissued after the purchase of discover (last fall). They are no longer on the VISA atm network although have some partnerships with ATM networks sometimes available in the EU. I have chosen to switch ATM cards so that I don't have to worry about finding a compatible ATM.
But the credit cards are still visa!

And to re-reiterate---or MasterCard. CapOne offered Credit Cards either network over the years, and does not matter which, as it seems they are handled in the same network. Credit cards are not ATM/debit cards.

Amex does not now issue cards to people in the Republic of Ireland so outside of more international tourist type of places there is little incentive for merchants to accept them. Acceptance in Northern Ireland is somewhat higher but by no means universal.