This can get complicated when you work shift! The rules are set out in S.I. No. 475/1997 - Organisation of Working Time (Determination of Pay For Holidays) Regulations, 1997. They are horribly drafted (like much of our statute book) and not very easy to plough through.
But, to summarize, the Act itself entitles you to, at the employer's choice, either:
- a paid day off on the public holiday
- a paid day off within a month of the public holiday
- an extra days annual leave
- an extra days pay
The Regulations set out that a days pay is one fifth of the normal weekly pay, including any regular bonus or allowances. In your case that would be 8.5 hours pay plus a prorata amount to reflect your Sunday premium allowance.
Neither the Act nor the Regulations spell out the length of the day off in lieu, but, by analogy, it should equally be 8.5 hours. If you were given a full 12 hour shift off in lieu, you'd be doing well! If you're paid for it, you should be getting the 8.5 hours pay adjusted for premia and any regular bonus.