Cook potatoes in boiling water about 20 minutes or until very tender; drain well and mash with skins on, adding milk and salt. While potatoes are cooking, melt butter in a large skillet. Add onion; cook 10 minutes, stirring occasionally, until very soft. Add cabbage; cook and stir for 5 minutes more or until very soft. Stir cabbage mixture and Kerrygold cheese into hot potatoes and season with pepper. Mound onto serving plates and make a well in the center of each. Pour a little melted butter into each well, if desired.