In a large bowl, beat the cream cheese, mayonnaise and mustard until smooth. Stir in the corned beef, onion, dill and salt. Using a 2 inch shamrock cookie cutter, cut out two shamrocks from each slice of bread. Spread 1 - 2 tablespoonfuls of the filling on the bottom half of the shamrocks, top with remaining shamrocks. Yields: 16 sandwiches.