Why stop at 3rd level? Why not charge fees/loans for 2nd level students too?
As Purple said, 2nd level education is optional. Also, those with a 3rd level education have higher earning potential than those that go to work straight after 2nd level (with some of their tax funding the education of 3rd level students.) In an ideal world 3rd level would be free, but Ireland's economy is far from ideal at the moment and was probably never really able to justify funding this.