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.