I'm travelling to Europe for almost a month (Ireland, England, Netherlands, Germany). Is there a way to get a phone plan which will work in all those countries? I don't mind buying a SIM card in Ireland (my first country), but I'd rather not have to buy a new one every time I switch countries. Data is most important to me - texting/calling is not such a big deal.
Three UK provides a Pay as You Go SIM which allows you to roam freely within Europe (and many other countries) without incurring any additional fees. They advertise this as "Feel at Home". You need to "Top Up" (add credit) to use the SIM, although a small amount of credit will come with every new SIM. This may be difficult online using a foreign card, so please investigate. Alternatively you can do it in a Three store, if necessary by using cash, or at a UK ATM.
To take advantage of the "Feel at Home" offer you need to buy a data "Add On" [otherwise you pay £0.01 / MB]. You can buy a data add on using your Pay as You Go credit.
Formerly Three offered a 2 GB add on for £5, but that seems to have disappeared from the Add Ons page. Instead the best bet might be the 12 GB add on for £20. Note that the "all you can eat data" (unlimited data) Add On does not apply to roaming, there is a 12 GB cap. That cap applies to both Pay as You Go and to contract customers when roaming abroad.
The Add On does not auto-renew but it is only valid for one month. So if you don't use the 12 GB, it doesn't carry over; it's gone 28 days after purchase.
Abroad, it's worth noting that you won't usually get 4G/LTE speeds, you are stuck at 3G speeds, which can be quite annoying if you are tethering. [They also say that tethering is not allowed when abroad, but it works fine for me.]
Three SIM cards can be purchased from any mobile telephone store in the UK or from certain supermarkets (Tesco I think). There's no need to register at the shop or provide any details, you can just pick up the packet and put it through the till.
Three do have an Irish brand (three.ie) but to my knowledge the Irish customers are not offered the same deal.
In addition to this, Three do have a dedicated internet-only SIM deal, which may be a better deal, but I have no experience of these or whether they work seamlessly with telephones (instead of with Tablets).
My knowledge of Three's pricing comes only from being a lifelong customer, so there may be better options around.