How to imessage someone with an Irish cell phone (iphone 6) that is currently in the Us. I'm in the US and have an american iphone

I have an urgent question. I recently met someone that is from Ireland. She is here in the US for a couple weeks. She has an iphone and she gave me her number. I also have an Iphone. I tried sending her multiple texts but the Imessage sign never appears and it keeps on sending as a normal text message (green). I tried calling, but I don't think she got an international plan, so her phone won't be active unless she's in Wi-Fi areas. When I text her iphone should I use international area codes as if I Was calling an international number or should I text it normally as people in ireland would? The structure of the number she gave me was 00353-86-xxx-xxxx. I did some research online and some websites say to use 011 and omit the two zeroes. Her normal phone number are without the two zeroes and without the 86. She's doing alot of traveling so I don't think she will be in alot of Wi-Fi areas. Any advice would help.

Use the number that she gave you - she's included the international dialing code for Ireland (Eire) which is 353!

Don't over complicate!