The simple answer is no. Where one party to a contract changes that contract without the consent of the other party, this is known as a unilateral change of contract and can in some cases lead to a breach of contract.
In general, an employer should try and agree any changes to an employment contract with the employee and preferably obtain their consent to any changes in writing. An employer is also legally obliged to write to the employee within one month of any changes being made.
In some instances, where changes have been made without an employees consent, and depending on the nature of the change, it may entitle an employee to resign from their employment and claim constructive dismissal. Such unilateral changes would need to be fundamental to the contract such as the payment of salary in return for work carried out by the employee. An employee could also have a claim before the Workplace Relations Commission under the Terms of Employment Act 1994 (as amended) which govern the provision of terms and conditions of employment. This was dealt with in our previous blog post.
Specific legal advice should be sought in all cases prior to any action being taken.