Section 60E(2) of the Employment Act says:
"(2) The employer shall grant and the employee shall take such leave not later than twelve months after the end of every twelve months continuous service and any employee who fails to take such leave at the end of such period shall thereupon cease to be entitled thereto:
Provided that an employee shall be entitled to payment in lieu of such annual leave if, at the request of his employer, he agrees in writing not to avail himself of any or all of his annual leave entitlement."
So, when your employer grants you leave, you have to take. If you fail to take, it will be forfeited. You cannot ask to be paid cash for it. It is only when he asks you not to take leave and you agree, then he will have to pay you in cash.