The maximum number of weeks of insurable employment that any employee can have in any income tax year is 52. However, if two paydays occur from the beginning of week 52 until the end of the income tax year, PRSI contributions will be paid on each payment as normal.
Remember, for PAYE purposes, an income tax year can have 53 tax weeks if there are 53 paydays in the year. Full details on how to calculate income tax in this situation are contained in Revenue's publication
IT50 PAYE/PRSI for Small Employers .