Prospective employers can ask to see garda checks, employment references, character references, exam results, passports and even medical information..
Credit checks aren't a huge stretch. Certain occupations require the employee to have a good financial standing so that they can't be seen to be corruptible -susceptible to bribery or theft.
But if your friend is an existing employee and this is coming out of nowhere.. I'd nearly be tempted to tell the employer to take a hike.