The only way to know for sure is to check online. Theres a good few Dublin Bike apps for smart phones which give you real time information on the status of every bike station. It changes all the time. But in general in the morning time all the bikes are gone, in the evenings all the stands are full. However people come and go all the time. So often you wait 5 mins and a bike or space comes free.
I found it pointless for commuting, but then it wasn't designed for that. Which is why they keep the bike stations away from the train stations.