As far as I can see, there are only seven places on the Irish inland waterways where you can park your boat at a diesel pump and fill up with marked gas oil (green diesel) from a seller who is legally allowed to sell it to you. All seven are on the Shannon; at time of writing (19 March 2013) there were none on the Grand Canal, Royal Canal, River Barrow, Erne (the part in the republic) or Shannon–Erne Waterway.
Behind this is a scheme [TinyURL http://tinyurl.com/bw29twk] introduced in 2012 by the Revenue Commissioners:
From 1st of October 2012, Section 101, of the Finance Act, 1999 (as amended by the Finance Act 2012) requires all Marked Fuel Traders who produce, sell, deliver or deal in, or keep for sale or delivery, any marked gas oil or marked kerosene to take out a Marked Fuel Trader's Licence. A separate Licence is required for each premises or place from which a trader operates.
Since last October, anyone selling or delivering green diesel, or keeping it for sale or delivery, should have applied for a Marked Fuel Trader's Licence, which costs e250 a year. Revenue requires regular returns of "oil movements".
Revenue has been publishing lists of holders of those licences and I've been reading them every week. At first, I could identify no Shannonside fuel retailers, so I checked with Revenue to make sure that the scheme did apply to private pleasure craft; it does, and I have heard that Revenue has already visited at least one boatyard.
The first Shannon fuel-seller to get a licence was Ciaran Fallon of Rooskey Craft & Tackle at Rooskey Quay. The other six licence-holders are CarrickCraft at Carrick-on-Shannon and Banagher, Emerald Star at Carrick-on-Shannon and Portumna, Hanley's Marina at Ballyleague (opposite Lanesborough) and Quigley's at Killinure on Lough Ree.
At time of writing, there is no licensed seller north of Carrick or south of Portumna. However, I may not have identified all the waterways fuel sellers correctly, and I would be happy to have my errors corrected. Updated lists of licensed marked fuel traders can be downloaded from http://www.revenue.ie/en/tax/excise/.