An agency is said to be irrevocable when the principal and agent agree that the agent's authority to represent the principal cannot be withdrawn for a specified period.
In fact, even when an agency power is agreed to be irrevocable, the principal can effectively revoke it, and bring the power of representation to an end. However, in these circumstances the principal will be liable to the agent for breach of contract.
An agency power is only truly irrevocable if that power was granted to guarantee some interest of the agent, such as a payment due by the principal to the agent: in such cases the agency power cannot be revoked.