I think it'll actually mean just 200 euro to the landlord.
It was meant to be 90% in 2019 but its 100% now. So interest of €4000 was to have 90% relief, that is €3600. Tax at say 50%, means tax bill of €1800.
Now, interest of €4000 has 100% relief, that is €4000. Tax at say 50%, means €400 more in deductible expenses resulting in €200 more for landlord.
People are making out like this is huge for landlords, its still fairly poor. What about LPT allowed as an expense? Reduce the tax on rental income???