One of the many links on this thread was to an article by an american economist talking about the effects of a house price crash. He made the point that rents could rise in such a scenario. It was based on the premise that investors defaulted on their mortgage and the banks siezed the properties. The properties, which were previously available for rent, were boarded up as the bank tried to sell them. As there is a crash, it is not easy to sell them, so they remain boarded up a long while, thus reducing the amount of rental stock out there.
However this scenario doesn't seem too likely to me in an Irish context. Some poster mentioned a bank scheme whereby they froze your mortgage and rented your house back to you at a much lower rate. I think banks will be reluctant to sieze properties, and look for other methods. Investors who try to sell themselves will either sell to another investor (meaning the property is still available to rent) or to an occupier (meaning that some other property will be freed up to rent).