What precisely do you mean by 50/50 joint assessment? You would be most likely best off by opting for joint/aggregated assessment rather than single assessment or assessment as two individuals. Joint/aggregated is the most beneficial for most couples especially when one is on 20% and one is on 42%. Single assessment works out the same except that any shortfall is only processed at year end rather than as you go. Assessment as two individuals may leave couples worse off and I've never understood why anybody would opt for it. See [broken link removed] and check out [broken link removed] to get an idea of the difference between joint/aggregated assessment and assessment as two separate individuals.
Sorry, what I meant is joint assessment, but I'm wondering do you HAVE to decide on one partner to be assessed?
If I'm the higher earner and chosen to be assessed, surely I would benefit more than my spouse if I was given more tax credits? If I had more tax credits than my wife-to-be, surely that would make me better off than her at the bottom of the pay-cheque?
I don't understand your points here. Maybe you could post specific figures for your situation? Have you read [broken link removed]? Regardless of how the credits and standard rate cut-off point are allocated at worst at the end of the year when you get your P60s and send them in asking Revenue to balance your affairs things will work out correctly and to your maximum benefit.
If I'm earning 43k and paying x in tax and my wife-to-be is on 26k and paying y in tax is there a chance that if jointly assessed, I would end up paying less than x and my wife would end up paying more than y or will we both end up paying the same amount of tax as normal?
If X and Y are the pre-marriage deductions then after marriage you (as far as I can see) will pay less than X + Y collectively (e.g. due to the €20.4K married increment in the standard rate band which only benefits you if you allocate all of that to you since your wife is below the 42% bracket anyway). Apart from that I guess it may depend on how you split the tax credits. Note that the apart from the married standard rate band increment of €20.4K there may not be that many credits that are transferrable since the basic standard rate cut-off or any unused balance of the €29.4K, PAYE credit and employment expenses credits are not transferrable. In many cases that would just leave the married person's credit (often split 50:50 I presume), service charges tax credit and some other common ones. I think that your personal tax bill would reduce while your wife's would stay more of less the same. However you'd have to crunch the numbers to be sure. Note that in the first year of marriage you stay on assessment as a single person and balance things up at the end of the year and the next year you elect for the preferred form of married taxation.