There are a number of rules for when an individual moves abroad. The rules depends on residency status. If he is non resident for tax purposes you will be taxed as a single person. Depending on circumstances you may be able to claim aggregation relief.
If he is tax resident he will be taxed in Ireland on his worldwide income. There are various reliefs available to mitigate against income being taxed twice. In this case you can be jointly assessed and share tax credits.