An apartment sitting idle should be a disaster - the higher rent obtained in the following years would need to be significantly higher just to break even in terms of yield.
I would've thought as full occupancy as possible at all times, i.e. renting out at market rates, would be the most profitable strategy.
Well it depends on where you are. Some places have different laws no idea about Dublin but some places protect renters from having the price spike up too quickly and have fixed rates once a contract is signed.
Edit: I of course have a subjective opinion on this from my point of view and where I am from the laws are heavily in favor of tenants so it is next to impossible to throw people out or hike up the price once an apartment is rented out.