The Shannon is the main focus of inland boating holidays in Ireland, particularly since it is now possible to connect through to the Erne waterway in the North. Lots of hire companies in the business. Narrowboats/barges not really suitable due to the open water of the main lakes, essentially.

Both the Royal Canal (...All along the banks of...) and Grand canal are, officially, open to navigation from Dublin to the Shannon but have little tourist trade except in one or two areas (Health warning: AS FAR AS I KNOW!).

There is some traffic on the Barrow (South East of the Republic).

The best source of info on inland cruising in Ireland is the Inland Waterways Association of Ireland, a voluntary body of enthusiasts. I'll post some links if I can find them

Regards
p.s. I lived in Athlone - near the mid point of the Shannon's course - for many years and sailed dinghies/small yachts there.