Medical experts usually say the average person can live for 4 to 6 weeks without food. The amount of time someone can go without water is much shorter, roughly a week (or less).
Your employer can force you to take leave at a particular time as long as they give you at least two weeks' notice. However, it is common for employer and employee to come to an agreement about the timing of annual leave. If you put in a request for leave, your employer has to have a good reason to refuse it.
If your employer does not give you a P45 you should ask for it. P60: Your employer must give you a P60 (pdf) within 6 weeks of the end of each tax year. It is a statement of your pay and of the tax, USC and PRSI deducted by your employer during the year.