2.5 hours is pretty tight for a separate connection, but you ought to be able to do it if your inbound flght isn't significantly late.
After you claim your baggage in the domestic terminal, follow the signs for the International Shuttle Connector. This free shuttle runs between the domestic and international terminals at ATL and takes about 15 minutes.
After you arrive at the international terminal you'll be able to check in and check your bags at the Qatar desks. Keep in mind that you must be checked in and your bags dropped at least 60 minutes before departure.