In Ireland, we use PR-STV for elections. However, vacancies are also filled by PR-STV, so if there is only 1 seat to fill in the constituency (near certainty), then it is filled effectively via IRV in the constituency.
This ofc benefits the 2 large parties, though often, they won't run a candidate if a family member of the deceased.stands (where the seat is vacant due to death) ... gotta love de facto hereditary seats. On county councils, there is a "co-option" process. Basically, the council itself fills the vacancy by majority vote. However, there is a tradition (not sure if legally binding) that the seat is filled by a person from the same party as a vacancy. My view on how to fill depends on the method. For single seaters, I don't see a problem with just holding a byelection using the normal method. This can give a slight power boost to that constituency if the government has a small majority. However, under PR-STV, that is unfair against the smaller parties. One option would be to fill on the nomination of the outgoing member. This has the advantage that it is simple, and should ensure that the vacancy is filled by a person similar to the outgoing member. The European Parliament uses this system. A slight modification would be to require that the member specify their 'backup' before the election, kinda like a VP in the US. Another option is to use the original ballots. In Australia, for their PR-STV seats, the ballots are reexamined after a vacancy and the results calculated a second time. However, no candidate who is still sitting in the parliament can be eliminated (i.e. you can't lose your seat because someone else resigns). This has some potential problems in the maths, but it should ensure that a candidate similar to the outgoing member is elected, while allowing the voters' choice to determine the replacement. I think that is a good idea, and it encourages a party to run extra candidates so that they have 'spares' to fill vacancies. This can help reduce the ability of parties to perform vote management. ---- Election-Methods mailing list - see http://electorama.com/em for list info
