If you pay your AVC via payroll, tax relief will automatically be applied in this tax year.
If you want to claim tax relief for AVCs off payroll or apply AVCs for tax relief in the previous tax year, or carry forward AVCs to next tax year, you need to make a return to Revenue (most likely Form 12 in your case).
Back-dated AVCs for last year must be paid and claimed from Revenue by 31st October in current year (might be a bit of wriggle room for online filing, but I wouldn't risk it).