We compute the involutive Heegaard Floer homology of the family of threemanifolds obtained by plumbings along almost-rational graphs. (This includes all Seifert fibered homology spheres.) We also study the involutive Heegaard Floer homology of connected sums of such three-manifolds, and explicitly determine the involutive correction terms in the case that all of the summands have the same orientation. Using these calculations, we give a new proof of the existence of an infinite-rank subgroup in the three-dimensional homology cobordism group.Theorem 1.2. Let Y and s be as in Theorem 1.1. Then, the involutive Heegaard Floer correction terms are given by dpY, sq "´2μpY, sq,dpY, sq " dpY, sq, where dpY, sq is the Ozsváth-Szabó d-invariant andμpY, sq is the Neumann-Siebenmann invariant from [17], [23]. Theorems 1.1 and 1.2 should be compared with the corresponding results for Pinp2q-monopole Floer homology, obtained by the first author in [5]. For the smaller class of rational homology spheres Seifert fibered over a base orbifold with underlying space S 2 , the Pinp2q-equivariant Seiberg-Witten Floer homology was computed by Stoffregen in [25].The proof of Theorem 1.1 is in two steps. First, we identify the action of J 0 on H´pR k q (or, more precisely, H´pR k qrσ`2sq with the conjugation involution on HF´pY, sq, using an argument from [5]. Second, we prove that, in the case at hand, the action of J 0 on H´pR k q 1 In this paper, by ras we will denote a grading shift by a, so that an element in degree 0 in a module M becomes an element in degree´a in the shifted module M ras. This is the standard convention, but opposite to the one in [14].