They can avoid employer PRSI (10.75% in most or all cases I think) on any employee contributions made by them or the employee as far as I know. Don't know of anything else offhand.
The saving in employer's PRSI only applies where contributions are paid through payroll.
If a member makes a lump sum AVC out of after tax income, he or she can claim a refund of income tax and PRSI, but the employer cannot claim a refund of employer's PRSI.