So I'm from the U.S. and I'll be heading to Europe for two weeks in a couple of months and I haven't really found a phone plan/SIM card that I think will work for me. I'm starting off in the Netherlands, but I'll also be in Ireland, Germany, France, and possibly Luxembourg and it seems like many of the phone plans I've seen don't include the Netherlands as a featured country. I was thinking of going through O2 in the UK with a 30-Day plan, but I don't know if I would qualify for the travel option that charge $1.99 daily for unlimited data abroad? Does anyone know?

02 "EE" or "3" offer bundles that you can buy to give Calls and Texts at base rate prices.. If you want Data you need to stick to getting Free WiFi wherever you can or a different operator for each country.

Or you can switch to T-Moble!

T mobile is EE in the UK and other European locations.. if you go down that route be ready for some rather iffy connection quality.

I've never experienced a connection problem in Spain, France, Portugal or Ireland.

T mobile is a major provider in the Netherlands. My son uses them with no problem.
Unless you get T-Mobile in the U.S. before you leave you are going to face roaming charges whichever provider you choose in Europe, for calls as well as data. Call roaming is nowhere near as bad as it used to be, but data roaming, assuming you can find a company that allows it, will soon rack up the costs unless you limit yourself to free WiFi, which is not always as available as some here would have you believe.
So either T mobile before you leave, or a new sim in every country if you are likely to need data.

Consider Vodafone too. See if they allow data roaming with a payg sim.

Make sure you read well the small letter about the 1,99$ a day for the unlimited connection! I'm not sure how the O2 defines Unlimited in terms of Gb, but there's definitely a limit (a few Gb)+ Vat+potential extra fees...I don't know, make sure they inform you correctly.
Good luck and have a nice trip!

Every time I see this topic, I think it's about using oxygen...LOL