In Germany there's a legal limit to how much the rent can be increased in addition to the time limit. And the limit depends on the location, so there are network effects. Additionally rent increases always have to be justified.

And even so tenants can sue against rent increases and occasionally win, resulting in no increase or a lower increase. There's even a Renter's Association that can provide legal aide to its members when there are disputes between tenants and landlords.

> Rent increases can only be performed every two years in Ireland from Dec 2015. And you must have 90 days notice of a rent increase. More info on this link:

Yep that's a new law and step in the right direction. But it doesn't really limit the amount. So landlords can just batch up 2 years of increase in one go.