IRV is not really a good voting system. It's better than plurality, but it still doesn't really allow viable third parties. If a third party ever catches on, they could steal first votes away from a major party, and cost both of them the election. Demonstration here: https://www.youtube.com/watch?v=7Q7rzqJ0YS8
I would recommend a condorcet method, or approval voting. Condorcet methods in particular have a lot of nice mathematical properties and are close to optimal with honest voters. I think it's the most likely to allow third parties to actually get elected. You can see some results of simulated elections with different voting systems here: http://rangevoting.org/BR52002bw.png http://rangevoting.org/BayRegsFig.html
They claim that just approval voting is the same level of improvement from plurality voting, as plurality voting is from monarchy. Based on the massive voter satisfaction index improvement. It really is crazy that we still use plurality voting.