Changing or varying the terms and conditions of a contract of employment can only be done with the agreement of the parties.
An employer is leaving him/herself open to a successful claim if he imposes changes to a contractual entitlement unilaterally.
It is worth noting that agreement can be express, implied, or by acquiescence. (See also amending the terms of employment)
What about variation of the terms of employment through the trade union negotiating on behalf of the employee? Generally employees will accept changes negotiated on their behalf by their trade union.
However a trade union cannot bind those members who have made it clear that they will not be bound by the changes-see Goulding Chemicals Ltd v Bolger , Irish Supreme Court.
Custom and Practice
Some contracts of employment will have terms of employment implied into them by custom and practice of the employment or industry. For this to happen the custom must be“so notorious, well known and acquiesced in that the absence of agreement in writing it is to be taken as one of the terms of the contract between the parties” O’Reilly v Irish Press 
Contractual Right to Vary
Many employment contracts will contain a term reserving the right to the employer to vary or alter the terms and/or conditions of the contract. However this does not give teh employer the right to make unreasonable changes and courts and tribunals will always look to see if the change was necessary and reasonable.
It is important to note that if an employee does not object to a change and works away under the changed terms he/she may be held to have implicitly agreed to the changed terms and conditions.
On the other hand an employee could argue that he/she was simply being co-operative and this did not imply approval of the change. The best way for an employer to counter this is to bring any proposed change to the attention of the employee; if he/she does not he cannot slip changes in “under the radar” and claim acquiescence by the employee.
It is worth noting also that where an employer is entitled in law to make changes to contracts of employment employees are still entitled to engage in trade disputes to attempt to bring about change. This is the case even in companies where unions are not recognised as the Labour Court can be asked by the union to investigate the dispute.
See also how to cut wages and salaries.
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Employers-How to Avoid Costly Employment Claims