Cook the macaroni according to the package directions. Drain the pasta and rinse under cold running water to cool completely. Drain well, put in a large bowl and set aside.
Mix the mayonnaise, salad dressing, onion, radishes and peas together in a medium bowl until well combined and evenly distributed. Pour the mixture over the macaroni and fold to combine. Taste and adjust the seasoning with salt and pepper. Serve immediately or cover and refrigerate until ready to serve.