I just tried filling the IE PLF.
When it asks "How are you arriving to Ireland" your answer is "Via Northern Ireland".
All travel by bus or train is via Northern Ireland so there is no need for separate options.
Thank You.
I finished filling passenger locator form and look what they wrote me at the end.
------Thank you for completing the Passenger Locator Form for entry into Ireland.
Your PLF receipt is attached with this email. You must present a copy of this receipt to airline staff or handling agents responsible for boarding aircraft or ferries.
You may also be asked to present the receipt to border management, immigration officials and other authorised parties.
If printed, the receipt must be kept in a presentable condition.