On the Grand Canal, M boats (boats with the letter M added to their numbers) were motor barges owned by the Grand Canal Company itself; E boats were engineering boats, used for canal maintenance and B boats (whether motor or horse-drawn) were bye-traders’ or hack boats, boats owned by traders other than the Grand Canal Company itself.
G boats were wooden horse-drawn boats, built to carry turf (peat) during The Emergency, which to the rest of the world was the second world war. Some of them [PDF] were built by Thompsons of Carlow, whose archives are now in the National Archives of Ireland; you can read about them in the Summer 2012 newsletter [PDF] of the Archives & Records Assocation.
The same organisation’s Autumn 2013 newsletter had an interesting article about Lagan Navigation archives in the Public Record Office of Northern Ireland. Unfortunately something has gone wrong with the links on its newsletter web page so the relevant PDF is not available at the moment.