There is no way a landlord in Dublin can get stuck with a tenant for 30 years. Rent increases are capped/set at 4% a year. The length of a lease is meant to be six years, but a landlord can terminate a tenancy at practically any time using a number of exemptions/loopholes in the law – to let a family member use the property, selling the property, refurbishing the property, etc. The rent increase limits should apply between tenancies too, as the landlord is legally required to tell new tenants the rent of the previous rent if they ask, but it is up to the prospective tenant to enforce this. At a time when demand for rental properties has never been higher, landlords can easily choose the tenants that don't ask these kinds of questions.
The root cause of high rents in Dublin is two-fold: competition for rental units is extremely high, which drives prices up; meanwhile rent is usually a function of the mortgage that exists on the property, which is determined by the price of the construction and/or the value of the land it's built on – and that's where you get massive amounts of speculation in Dublin. Land in Dublin is so expensive that it drives up the cost of everything else along the chain.
Most of the vacant units in Dublin are old, and in terrible shape. They're largely owned by people who either don't have the resources (money/energy/time) to improve them so they meet minimum standards, or they're owned by people who know they can let them sit for five or ten or twenty years and sell them for a profit then. The site they're on is nearly always going to increase in value, regardless of the state of the property.