A word of warning, some colleges are just a cover for getting migrants into the country illegally (they come in on a student visa and then disappear when their visa is up!). So be careful, because you could find your address being used for them to open up bank accounts etc. Also, they tend not to bother attending classes and can end up dossing around your place all day, using up electricity, heating etc. I would advise to take students from EU countries only.
That is true of some colleges but thousands of non-eu students enter the country every year for registered courses in RCSI, UCD and many others. Take what nationality you are comfortable with but be wary of some language schools. Ask around - a friend of a friend etc. can normally recommend a college to use.
What I meant was that language schools which don't have any students from EU countries are probably a bit dodgy, so just be careful of taking people from these colleges. If someone is attending UCD or somewhere else that is well known and reputable then I agree they would obviously be genuine students.