Wash, peel, and cut the potatoes into cubes. Place them in a pot of salted boiling water and cook for 10 minutes.
Remove the potatoes from the water and add the cabbage and onions to the pot. Cook for 5 minutes until softened. Drain and set aside.
Mash the potatoes, then stir in the shredded cheddar cheese.
Heat a skillet over medium heat.
Lay the tortillas flat on a clean surface. Spoon a dollop of Thousand Island dressing onto each tortilla and spread it evenly over the surface.
Spread the mashed potatoes over one half of each tortilla, pressing gently to flatten. Top with sliced corned beef, cabbage, and onions. Fold the tortillas in half.
Pour ¼ cup of vegetable oil into the skillet. Carefully place the folded tacos into the hot oil and fry for 30 seconds. Then flip and fry the other side for an additional 30 seconds until browned.
If desired, garnish with chopped chives before serving.