A cruise down the longest river in the British Isles, the River Shannon which terminates in the sea past Limerick city, is the most popular one. It has 3 major lakes, Lough Allen, Lough Ree & the largest, Lough Derg. A couple of miles downriver of the town of Athlone is the monastic ruins of Clonmacnoise.
You cab hire a cruiser and do a holiday or you may be able to hop on a short trip from Athlone for example.
Alternatively you might look into taking a barge trip on either of the canals, the Grand or Royal which terminate in Dublin and go for quite a length inland connecting with other major river systems or down the River Barrow in Wexford.
http://www.cruiseireland.com/ireland-boating-holidays-map.htm about 6 years ago