"IRS" stands for "Increasing Returns to Scale". It means if you double all the inputs, you more than double the output.
I first worried that US readers might think "IRS" stands for Internal Revenue Service (the US tax people). Then I realised that I want it to stand for that too.
Staghunt is a simple game with two players. Each player alone can catch one hare each; both players together can catch one stag, which they share. Staghunt has two Nash Equilibria: both hunt hare alone; both hunt stag together. (If the other hunts hare you want to hunt hare too, because you can't catch a stag alone; if the other hunts stag you want to hunt stag too, because half a stag is better than one hare.)
Staghunt exhibits increasing returns to scale technology, because two hunters producing one stag is more than double the meat of one hunter producing one hare.
Staghunt is also a model of society. We gain by cooperating, but it only works if we trust others to join in. It's different from Prisoners' Dilemma, the alternative model of society, because Prisoners' Dilemma has only one (bad) Nash Equilibrium, and so needs a Leviathan (which can be interpreted many ways) to enforce cooperation.
Staghunt is also a model of a "network externality". The individual who joins the network (hunts stag, buys a telephone) creates a positive externality for others already in the network, and increases the incentive for others to join the network. (That second effect should strictly be called "strategic complementarity", or "positive feedback", because it is not an "externality" in the strict sense. See my old post for the distinction.)
So Staghunt is a very good model with which to examine Brad DeLong's recent thought-provoking post.