The default position for a married couple is to be taxed jointly.
If you have been taxed separately either one of you elected for this at some point, or you never informed Revenue that you were married.
If it was the first cause, the one who requested sep. treatment would have to contact Revenue and cancel the request. If the second, depending when you got married, you could be eligible in the year of marriage for some tax relief.
If you both have Paye incomes and this has been going for for a number of years, you should look at the past years' joint tax position in case you could be due a refund.