iPhone se use in Ireland

There's a 3 ways you can use it there.

1. Contact your cell carrier and inquire about any roaming plans they may have for Ireland or the U.K (it may be bundled in). Roaming allows you to use your current plan and service with a local Irish carrier by paying a fee to your carrier. This has the benefit of using your home number, so people at home can still cal and message you while you are in Ireland.
2. If your iPhone is unlocked, you can get a sim card from a local Irish carrier. This would give you an Irish number, and service. so anybody that does not have the new number would be unable to call you or message you.
3. The third way is Wifi only. While connected to Wifi, and having Cellular turned off via Airplane mode, you can make and recive Facetime audio calls, and send or recieve iMessages to and from other Apple devices. But will not be able to make or receive calls or send or receive SMS messages to and from non Apple devices.

I have never been to Ireland but most countries have short-term plans where you buy a sim card for 30 days or so and get telephone service plus a GB or so of data. I will let you do the work researching that online. I had it set up on my 4S in Germany last year. We got a real deal on SIMs and I think we got 5 SIMS with a month on each for about 15 euros total (a real deal!)

Even if you can't get telephone service, you can still use wi-fi and it is a lot lighter than a notebook or iPad. I downloaded a bunch of offline apps such as offline maps (Maps.me) and use my iPhones when traveling.