Yes, you need to file in ROS if you are abroad. Its a messy procedure to get it operational. They send you two separate mailings before you are operational and the programme itself is tempermental.
Strictly speaking, as you are non resident, your tenants should be withholding tax on your behalf, but I think the Revenue overlook this if you make good contact with them.
For filling up the form you need to do your accounts separately and then enter in Ros... rent, expenses and then capital allowances.