In a food processor, whirl cream cheese, butter, parsley, and horseradish to taste until smooth and pale green; scrape container sides as needed.
Lightly oil a 3- by 5-inch loaf pan (about 2 1/4 cups to the rim). Line with 1 layer of corned beef slices, neatly overlapped from 1 rim across bottom to the other rim (with ends extending over rim). Line ends of pan with corned beef slices, overlapping the bottom and pan rim.
Dot 1/3 cup cheese mixture onto meat in pan bottom; gently spread level. Cut 2 or 3 corned beef slices to fit pan, and lay neatly onto cheese mixture. Repeat to make 3 more layers, ending at the rim with cheese mixture. Cut 2 or 3 corned beef slices to fit pan and lay on cheese mixture to cover. Fold meat at pan rim neatly over filling; pat gently to make level. Wrap airtight.
Chill until firm enough to slice, at least 4 hours or up to 2 days. Unwrap terrine; invert a flat plate onto pan. Holding plate and pan together, turn over and lift pan off. Pat loose pieces of corned beef back in place.
With a sharp knife, cut terrine into 8 to 10 equal slices. Serve with baguette slices on plates.