An interactive map showing the total number of athletes from each country racing in 2012 Ironman events around the world.
It's easy to retrieve a lot more detail from my Ironman database, so in addition to the number of athletes, the following table also lists the fastest, average and slowest finish times from athletes of each nation across the entire Ironman series.
Another way to consider the number of competitors from each country is as a percentage of their population. Being a minority sport percentages are small, even in Australia - unsurprisingly the country where Ironman is most popular - only 0.02% of the population participate.
Note that as this data is based upon the country information listed for each athlete at each race athletes racing multiple Ironmans within the year will be counted multiple times.