NI Water Ltd is a State owned private company with a monopoly of supply. Before privatisation, Northern Irelanders paid for their water through their local rates, and privatisation was seen as a way to separate the function and build revenued (by charging for water directly) with a view to updating the antiqated system. It became a political issue at the local elections, and Northern Irelanders voted in parties opposed to the charges. The investment in new infrastructure never took place on the scale needed, and the result is the system failing. The Buck stopped at the ballot box.
Does not explain the under investment and failing water supplies in England and Wales though.