Pour the oil into a large pot or dutch oven until it reaches about an inch up the side of the pot. Heat it over medium heat to a temperature of 375 degrees F.
While the oil is heating up, prepare your egg rolls. Set up a station with all of your fillings: corned beef, green cabbage and cheddar cheese. Starting with 1 egg roll wrapper, place a few slices of corned beef, 1 slice of cheese, and about a tablespoon or two of cabbage onto the lower 1/3 of the wrapper. Roll the egg roll up by rolling the ingredients into the wrapper away from you, fold in the sides about halfway through the roll, and then continue rolling until completely in tact. Taking a bit of egg wash, seal where necessary. Place it on a plate, and continue with the remaining egg roll wrappers.
Once the oil is hot, add one of the egg rolls and fry for about 1 minute on each side. Do this first step with only one egg roll so that you know the oil is at the right temperature. Remove the egg roll from the oil, and transfer it to a plate that has been lined with a paper towel. Continue this step for the rest of the egg rolls. At this point, you may add 2-3 egg rolls at a time. Just make sure to not overcrowd the pan.
Serve the egg rolls immediately with homemade beer mustard, and enjoy!