First check out the 'Organisation of working time act 1997 '' Below is an excerp which may not come out very well as its copied . You will find it at http://www.irishstatutebook.ie
19. an employee shall be entitled to paid annual leave (in this Act referred to as "annual leave") equal to—
( a ) 4 working weeks in a leave year in which he or she works at least 1,365 hours (unless it is a leave year in which he or she changes employment),
( b ) one-third of a working week for each month in the leave year in which he or she works at least 117 hours, or
( c ) 8 per cent. of the hours he or she works in a leave year (but subject to a maximum of 4 working weeks):
Also, don't forget to factor in the bank holiday entitlements while you are on leave. e.g. if there are three bank holidays during your maternity leave you are entitled to an extra three days leave!!!