The usual questions:

Do you remember any of the lyrics? Please post what you remember to help with internet searches.

When did you hear it or see it in written music collections? If it was in written music collections, what sort of collections are you likely to have seen it in?
Was it recorded? Did you hear it on the radio, see it on TV, hear it on vinyl, CD, digital, YouTube, etc?
Did you hear it live?
Singer(s) male, female, a group, solo?
Musicians? Harp (I'm assuming) but any other instruments.
Lively, haunting, slow, fast, dance tune, etc, etc?

Is it possible the name of the song is incorrect? Fingal, Donegal, Finnegan, etc etc.

Any more information would be helpful. At present, nothing useful comes up on Google search for "Finegal the Harper".