Nevertheless, my initial question remains - does anyone know if it is a legal obligation of the airlines to provide potable (ie drinkable) water?
I'm not sure about licencing laws on board Irish a/c, but under Irish law any restaurant with an alcohol licence is required to be able to provide drinking water free of charge to customers. I've used the rule on occasion to straighten out sniffy waiters who prefer to see you pay for overpriced bottled water.
Someone would have to check out the regs regarding drinking water on board a/c. It might prove interesting.
However, the water used for flushing loos in my house is the same stuff that comes out of the taps, and I drink it and feed it to my children without worry.
The water from your kitchen tap is almost certainly MAINS water, not TANK water.
I would avoid drinking the water from my bathroom taps. Aside from anything else it's usually tepid from being stored in the attic - and you can easily taste the difference from fresh mains water. Bathroom (tank) tap water is only fit for bathing in, or brushing teeth, or flushing loos.