If an employer wishes to have a mandatory retirement age, then it should be set out in a contract of employment. By stipulating a retirement age at the commencement of the employment relationship, you are putting the employees on notice from an early stage that the company's retirement age is 65 (or whatever age is set for the business).
The entitlement to retire an employee at a retirement age is usually dependent on a contractual provision providing for it. So, if the employee's contract does not provide for retirement at a given age, then the situation is more problematic for the employer and the employee could remain employed by the company indefinitely. If forced to retire by the company the employee could assert unfair dismissal or age discrimination.
If a contract is silent, then generally there is no date at which an employee can be forced to retire. Previously Section 2(1)(b) of the Unfair Dismissal Acts 1977-2015 excluded employees who were over the age at which they could claim a state pension. This was amended in 2004 and failure to clearly establish the normal retirement age may lead to claims of unfair dismissal.
In an Employment Appeal Tribunal (EAT) determination (O’Carroll v Sovereign Security Limited (UD1131/2009)) an award of €25,000 was made to a security guard who alleged he had been constructively dismissed after his employer reduced his hours when it was discovered he was past the state pension age. The EAT noted in particular the lack of any mandatory retirement age in his original contract of employment. It was also clear from the evidence that no mandatory retirement age could be shown to exist within the company.
Even in the absence of an express contractual retirement age, custom and practice can provide sufficient evidence of the existence of a retirement age. An example of this was where the Equality Tribunal upheld a compulsory retirement age of 65 despite the lack of a written policy or contractual clause on the basis of a well established practice of compulsory retirement and entitlement to pension at age 65 (Paul Doyle v ESB (DEC E2012-086)).
If a retirement age is not clearly established in the contract or the employee handbook the onus will be on the employer to show a well established practice to avoid an unfair dismissal finding or entitlement to redundancy.