STV isn't in itself a proportional representation system (and it isn't the system used in Ireland).

The idea of STVI isn't so much to ensure representation of minorities but rather to make less likely the situation where a party that most people detest will slip through and win in a first-past-the-post election because the votes of the people who detest it is split between other parties. (For example neither Thatcher nor Blair ever got a majority of popular votes.)