Your employer can require you to provide a medical certificate (from your GP or family doctor) when you are on sick leave. For example, you may have to provide a medical certificate if you are out sick for more than 2 consecutive days. The medical certificate should state the date you are likely to return to work. If you are likely to be out sick for a longer period, your employer may require you provide weekly medical certificates. Please be advised that certifiaction to work is not covered by Medical Card.  

The Citizens Information website contains a wealth of information on Sick Leave and Sick Pay and Illness Benefit.