JK, rent is based on supply and demand. With the level of property investors over the last few years supply has grown rapidly. With the booming economy of the past decade or so we had a very large level of growth (foreign workers, Irish workers returning etc) in demand, so rents increased.
Over the past couple of years however rent increases have slowed dramatically and in some cases fallen (I had my rent decrease from 2005 to 2006, only a small amount, but given inflation etc. still worth noting).
There is no magical formula that means rent will always increase. It always comes back to supply and demand. If the economy slows the number of people looking to rent may fall (more people leaving the country again, less coming in from other countries) and rents may/would slide accordingly.
None of us can predict this, no more than we can a property crash/correction. I'm not saying rents will fall (over a medium to long term), but you certainly can't say with certainty that they will rise!
So, as for coments like "rents don't go down", I'd like to be the second of many to disagree, with a personal example to back it up.
To the OP, you will find people who fall strongly on either side of the fence (bullish like JK, bearish like.... well, half the posters on AAM). Neither side is correct (until we can look back) so you need to form your own opinion on what to do. Becoming a landlord comes with a lot of risks attached to the potentially (very) large returns. Other forms of investments can also provide large (though seldom in the league of property in the last decade) returns for high risks or smaller returns for less risk. If you have one property you increase the risk of holding onto a second by having all your eggs (investments) in one basket (non diversified portfolio). Once you can understand the potential risks/returns, you can make an informed decision, but do be sure not to get swayed by the strong beliefs of another, whether that be on the bullish or bearish side.