In mixing bowl, break up canned corned beef with fork or cut fresh corned beef into shreds.
Add remaining ingredients except buns; mix to combine.
Cut buns in half horizontally and scoop out part of inside bread (can be saved and used for bread crumbs), leaving about 1/2 inch edge on crust of roll.
Spoon corned beef into bottom half of rolls, pressing into cavity.
Top with top half of roll.
Heat on paper plates or napkins by cooking, uncovered, until steam appears when top of bun is lifted: