Your tax liability on rental income (if you are non-resident) is 20% of the rental profit, which is the rent less mortgage interest and rental expenses. However, unless you use a rental agent, your tenants will be obliged to withhold 20% of the gross rent and pay this over to the Revenue. When you do your tax return, you should then be due a tax refund.
If you rent the property and sell it without having returned and lived in it, you will be subject to CGT on any gain (at 20%). However, the proportion of the gain subject to tax will be reduced by the proportion of time you spent in the apartment (plus an extra year) over the total ownership period.
If you return to live in the property, you will not be subject to CGT on a later disposal. Alternatively, if a "dependent relative" or widowed parent moves into the property, their period of residence will be treated as if you were living there.