In this case as it's the world cup, fifa give a certain amount of tickets to each country to sell. So the fai is Irelands football association only got 1700 tickets to sell. You have to have an account with them to buy one. The other 40,000 either went to the Australian football association or more than likely half of that 40k given to officials (ie FIFAs mates). You can absolutely try get tickets from else where and they won't know who you support. It's not a way to know who supports who, it's a way to make the tickets available for the countries that are playing in that match. Makes more sense for Irish and Australian fans to get tickets than a french or German who aren't playing that game.

Do you know if the extra tickets on sale? My sister is there and wants to get some. Supposedly from the original ~42,000 Irish supporters were only given 1700. I get it's Australia and they are hosting it, it being the first game and that. But that's terrible allocation. Major tournaments should be 50/50