Day 1-28 — the standard charge
Ireland has no early-payment discount — the amount printed on the notice IS the standard charge, and it only ever goes up. From the date the FCN is served you have 28 days to pay it (€40 on many parking FCNs, but amounts vary by council and offence — go by your notice) or submit a written representation to the issuing authority. The day of service counts as day 0 — count from the day after.
Day 29-56 — the charge rises by 50%
On day 29 the charge increases by 50% (a €40 parking FCN becomes €60). You can still pay or maintain a challenge during this window. There was never a discount to lose — this is a late surcharge, so the only way the extra comes off is if your representation succeeds.
Day 57+ — summons risk
After day 56 the issuing authority can refer the matter to the Director of Public Prosecutions, who issues a District Court summons. The court can impose the original fine plus court fees, and (for Garda FCNs) penalty points.