You haven't explicitlyu stated this, but I assume that your wife is not being paid maternity pay by her company.
If so, your overall tax bill, as a couple for the year will be less, as you are earning less.
The question is - when do you see this benefit ?
I dont know how you have split your tax credits, but her tax credits will not automatically go to you unless you ask the Revenue to do this, so you will not see an automatic reduction in your tax bill.
If you dont do this, you will get any tax back at the end of the year, but I'm sure you
would prefer to have it now.
I would suggest that you ask for all her tax credits to be transfered to you.
If you do this, she may pay more tax than usual if she goes back to work before the end of the year, so be careful.
You should also make sure that you do an end of year tax return, with all your
medical bills to ensure that you get any rebate due.