In a 2 quart saucepan over high heat, bring potatoes and enough water to cover to a boil. Reduce heat to medium low. Simmer covered for 30 minutes or until potatoes are tender; drain. Let stand until cool enough to handle. Meanwhile, finely chop corned beef and onion. Finely chop cooled potatoes. Place chopped ingredients in a large bowl; toss to mix well. Season mixture with a little salt. Shape into 6 patties about 1/2-inch thick. In a 12-inch skillet over medium heat, melt 1 tablespoon butter. Cook 2 patties until brown on both sides. Remove from skillet. Keep warm. Repeat with remaining patties.