I don’t think that there is a specific form for opting for separate assessment.
According to Revenue’s website, a verbal claim for this method of assessment will suffice.
If you have not already done so, you should advise Revenue of the date of your marriage, the PPSNs of you and your spouse and that you wish to be separately assessed. You should also advise of any other details, such as your current address.
You can do this by telephone call, email or letter.
Unused credits and rate bands of one spouse are transferred to the other spouse by end-of-year review.
Regarding the year you were married, Revenue may already have details of your earnings from the Forms P35 of your respective employers, but a telephone call or email to your tax district will clarify whether you need to submit P60s