Although Beachcomber was specific, in the original question, about solar heated water supply a few posters have introduced photovoltaic power generation as a topic. Coming from a place more endowed with sunshine (and thus less endowed with water) than Ireland, my experience may be of little help but, here goes.

New England (Australia) has a rather frosty winter (meaning very clear skies) and a warm summer. My house is constructed to allow passive solar heating, which means that there are a few weeks in winter where, what architects call "the comfort zone", extra heating is required. The hot water supply is a closed-cycle thermosiphoning solar panel, meaning that the glycol/water mix that circulates through the panel and the heat exchanger in the storage tank is isolated from the potable water. Open-cycle panels allow the potable water to pass through the solar panel and the experience here in winter is that deep frosts can burst the piping in the solar panel unless you drain it overnight. Extra plumbing (read "expense") and the chance of unexpected frosts causing damage and loss of precious water mean most people with solar hot water systems here use closed-cycle thermosiphoning solar panels.

Over the years I've averaged no more than a week of consecutive cloudy days, meaning that the water temperature (usually 80 degrees Celsius) will drop to 50 by the end of that time. The storage tank has an "Off peak" electric element that, if I switched it on, would automatically heat the water to 72 degrees Celsius; I don't bother. If I lived in a cloudier environment I'd probably consider a heat pump; electrically driven (and thus more expensive to run), these can extract heat from air at temperatures as low as about 5 degrees Clesius to heat air or water.

My father (in Adelaide, with a more "Mediterranean" climate according to the geographers) used a home made set of solar panels to heat his swimming pool. The piping was all "poly pipe" of the type used in agriculture (I think Americans call it ABS piping), with easy jointing, and mounted on the roof. Thermosiphoning won't work if the storage (the swimming pool) is below the panels (on the roof of the house) but the pump used for filtration and treatment of the pool water also pumped through the solar panel. He gave it to me to install in my house as a space-heater but I'm not installing it until I can reduce my power requirements.

Which brings me to photovoltaics. I have enough north-facing roof to install 40 odd square metres of such panels at the right angle (pun intended) to maximise efficiency but I have concerns about electrical storage; the batteries are special, expensive and not particularly friendly to either the OH&S nor the ecological environment. So I wait for the local electrical power suppliers (often called "No Power" in the past) to change their policy and accept people like me feeding excess supply into the grid and offsetting my bills.

In an Australian rural environment (read 'bushfires') distributed power sources across the network can be a problem. When the main grid is disconnected (accidentally, because of breaks in the line, or deliberately, to ensure safe working below the line in smoke that may allow arcing) you don't want the lines still alive because of characters like me still pumping power into the grid.

Sorry I know nothing about Irish tax incentives but the above may be helpful.

Cheers, Rowan