Since 2012 Revenue stopped issuing paper tax certificates to individuals. You must register for Revenue's myAccount online (see attached guide) and you can add, edit details online as well as view your current tax situation
Your tax credits are given to you for a full tax year. So, whether you start work in the first week of the tax year or six months into the tax year, in most cases you qualify for a full year's tax credits.
In exceptional circumstances, e.g., relocated to Ireland late in the tax year, you maybe given tax credits on a week by week or month by month basis depending on your personal circumstances.
As tax deductions are spread evenly throughout the year under the PAYE system, the total due is divided into 52 weekly/12 monthly amounts, depending on frequency of pay. Your employer grants these credits against your gross tax to arrive at your tax payable.