OP, she has lived illegally in Spain for several years with no problems.
Why can't she live illegally in Ireland?
I know many non EU immigrants living in England illegally, some for 20 odd years.
Why can't she do this? Nothing's actually going to change is it? She will carry on working and earning a full time wage in what is meant to be a 20 hour a week job (whoops!!), seeing you, travelling and doing whatever else she does. Except going to college or whatever she's doing now that she hates doing, of course.
So my advice is don't marry. Just continue as you are. If she finds that too "upsetting" and "heartbreaking" then just point out that she's done it before and nothing happened. Therefore nothing will happen in Ireland.
If she's really genuine and really in love with you and really innocent and pure as you put it, then she won't have a problem with any of this.
If she does have a problem, well, then maybe she isn't really in love with you at all.
What's she doing with all her money by the way? She's not earning badly for a student.