There are boats available for hire from Portumna on the northern tip of Lough Derg and from Banagher, about half way between Portumna and Athlone. The river on the southern Shannon is mostly wide and slow with just two locks between Killaloe and Athlone - locks cost €2.50 to pass through at the time of writing. There's also the swing bridge at Portumna, this bridge needs to be open for cruisers to pass through and opens at fixed times through the day. The locks on the lower Shannon are manned and the lock-keepers will be happy to help you through the locks, and offer any advice you need on your destination.
Navigation on the lakes and rivers is easy, there are green and red markers that outline the cruising channels, they're painted white on the side that you need to be on. The markers are accurately marked on the navigation maps that will be on board your boat when you take over and full tuition is provided.
There's free mooring at every town and village along the Shannon and most mooring is within 5 minutes walk of the village or town. Mooring is paid at Athlone as it's supervised, the last time we were there it was €12 for the night including power if required.