I mentioned the other day that extensive searches of tinterweb had failed to find any data on the heights of overhead power lines above the Shannon and that I had been forced to resort to the telephone.
I am pleased to report that the ESB expert duly rang me back today, and further pleased to report that he had himself measured the height of every cable over the Shannon. Oh joy, oh happiness, I thought. But not for long.
The ESB, it seems, does not reveal the actual height of power lines above the water. This, if I understood the reasoning correctly, is because the water level varies and a boat-owner might not understand that, hit a line with a mast and then sue the ESB [where “ESB” means “electricity transmission or distribution operation”]. I have been told that a boat-owner in coastal water did just that (presumably between the mainland and an island) and that the lawyers have advised ESB to take no further risk; I would welcome information about the incident.
So, if you want to find the height of a power line, you’ll need to polish up your trigonometry (and then relate the height to Ordinary Summer Level). But the ESB does supply the information to one organisation that makes charts: the United Kingdom Hydrographic Office. That’s OK for coastal charts, but the UKHO (to the best of my knowledge) has not surveyed the inland Shannon since 1839, and I don’t think its inland charts have been updated since then. It is possible, of course, that the UKHO supplies the data to other (electronic or paper) chart-makers; I have emailed them to ask.
The quest continues again. More info when I get it.
Irish waterways history 
hiliarious and/or terrifying… surely though, there is a greater risk of them being sued for not publishing ANY information at all?!
Also, isn’t the standard thing in this sort of situation to have some kind of gradated marker posts in place, that let sailors see the height at any given tide?
It may be that non-feasance is better [ie legally more defensible] than malfeasance here, as it is with road repairs and such.
Waterways Ireland has airdraft gauges, such as you describe, for most if not all of the bridges on the Shannon.
bjg