www.rsa.ie
This website might be of some help to your friend. I think all driving instructors have to pass a test now before they can start working as a driving instructor.
More here. It costs €550 to take the three ADI tests, and a biannual registration fee thereafter. You would need to consider the demand for instructors in your area.
It is estimated that there are 2000 driving instructors working in Ireland at present.
Many people want their lessons outside work hrs - therefore he could keep his steady job and do the instruction in evenings, w/e and use a few of his hols until he gets a feel for the demand and the job itself.
I agree with Joe Roberts. If a driving instructor is willing to be available at evenings, Saturday and Sunday mornings etc he/she will get a lot of customers. There is a market there