I don't think the biological environment of 2018 would be lethal to her, if she gets with normal preventive care (e.g. vaccines)... but unwanted government attention could result in a fate that's worse depending on how the government feels about time travel.
Big problem: she's stateless. A lot of nations have nationalized or single-payer healthcare, which means a person who's not in the system, or who has never had any of these vaccines, will be very, very unusual, and will raise a lot of questions by a lot of people.
They would really dig into how she could possibly have been missed by the health system, out of a loving desire to stop others from being missed,vand to justify the cost (to the government) of the full regimen. That would only raise even more questions, which would escalate and come to notice of officials less interested in her welfare.
Stateless means is an illegal alien in every country in the world, and a citizen in none. It would be a challenge to establish a nationality for her. Each of these ideas requires her looks fit the story. Might be hard for an Irish woman.
- Find a country whose records for citizens of that age are very poor due to extended civil disorder, and for which it would be common for an adult woman to just pop up with no records whatsoever. Assimilate enough to pass as a native and ask for the now-stable government to document you.
- Join one of the streams of refugees somewhere around the world, after assimilating enough to pass as one.
- find a country that has path to citizenship for illegals who arrived as children, e.g. USA DREAM Act.
- Fabricate some sort of "Kimmy Schmidt" style story where she was born in captivity of some creepo.
- Convince a relevant government that her parents were "off grid" freaks who didn't register her birth or do any of the usual government things. Pass a DNA test that proves definitively that she's from around here.
- Game immigration laws like those of the UK where they can only deport a person if they can prove their original citizenship (which they'll never find for her). But "permanent limbo" isn't a very good existence.
- Game the programs some countries have to help stateless people.
- Be hugely political to curry favor of a semicorrupt country like Russia whose leaders freely hand out citizenships to buddies.
- time travel to when time travel is a normal thing, and "land" (clear immigration) in the normal way.** If you're coming from the past, get a lawyer. Thanks Pere.
And then you could get on their national health system and get your care, because these things would quiet the question of why her history is a black hole and why she's never had a vaccine.
Another option is go to a country where the medicine is quite good, but very disorganized, balkanied and inefficient (and they like it that way). Where it's rather common for an anonymous person to walk into an urgent-care with no identity or insurance documents whatsoever and ask for a-la-carte care and peel off $20 bills to pay for it. I'm speaking of course of the United States. Doctor's offices keep their own medical records and don't share unless asked by another office. You could keep your own medical records and get 100 vaccines at 100 urgent care's or free clinics. There, the risk is missing an important vaccine.
** my greatest aspiration is to travel to England, find where Immigration enforcement is working, step out of a blue Police Box that wasn't there before, and go "pardon me, are you with Border Force? Would you mind--" and pull out my passport.