Irish drugmaker Mallinckrodt Plc agreed to pay $100 million to settle claims that its U.S. unit illegally boosted the price of a rare autoimmune drug by 85,000 percent and bought the rights to a much-cheaper competitor to keep it out of the American market. The unit monopolized the market for years to increase sales of its H.P. Acthar Gel, typically used to treat multiple sclerosis, infantile spasms and other life-threatening diseases, the Federal Trade Commission said Wednesday in a statement.