If you wish to explore the Lower Shannon, Carrickcraft has its marina station at Banagher, County Offaly. From there, you can sail further south into the mountainous region of Lough Derg. You can cruise Lough Derg and make stops along the way at several places including Meelick Quay, Portumna, Terryglass, Dromineer, Garrykennedy, Killaloe, Scarriff and Mountshannon. Carrickcraft says: "We can't promise to match any price on the Shannon--our boats aren't the same and comparison is impossible."
If you wish to cruise from Portumna, Emerald Star has a base there and claims to be Ireland's number one boating holiday company on account of their 40 years of experience and large fleet. They claim to offer more one-way routes than any other operator, support when you need it 24/7 and English speaking staff.
Silver Line Cruisers is situated at Banagher, Co. Offaly, between Lough Ree and Lough Derg, one mile from the Grand Canal. The company provides free car parking and the free loan of a 13 foot fishing dinghy to explore shallow waters and picnic on islands. The company gives a total price including your ferry crossing for all passengers and cars.