In small saucepan, combine Kerrygold Pure Irish Butter, capers and raisins; heat over medium heat just until butter simmers. Remove from heat; let stand to soften raisins and cool slightly, about 5 minutes. Process mixture in blender or small food processor until well blended. Remove to small bowl; keep warm until ready to serve. Preheat broiler; position oven rack so surface of fish is about 5 inches from the heat. Brush both sides of salmon with melted butter; season with salt and pepper. Line shallow baking pan with aluminum foil.
Arrange fish on pan, skin side up. Broil until skin begins to brown and chars slightly, about 2 to 3 minutes. Turn fish; broil just until fish is opaque throughout, about 4 to 5 minutes. Meanwhile, place spinach in large skillet. Cover and heat over medium-high heat until spinach just begins to wilt, about 4 to 5 minutes, stirring occasionally. Season with salt and pepper.
To serve, place equal portions of spinach onto 4 warm dinner plates. Place salmon fillets over spinach. Arrange potatoes, if desired, alongside. Drizzle with Caper & Raisin Butter. Refrigerate any remaining butter to use with cooked chicken or other seafood.