Appearing before Belfast magistrates on 20 April, the Guinness subsidiary pleaded guilty to five breaches of section 32(1) of the Radioactive Substances Act 1993.
The company was fined £5,000 for failing to keep the source secure, and £500 on each of four further charges related to looking after the source and keeping records of its whereabouts.
The americium 241 source was found to be missing in September 2002 during a routine inspection of the firm's Belfast plant by the Industrial Pollution and Radiochemical Inspectorate, part of Northern Ireland's Environment and Heritage Service.
The source, used in a gauge to monitor liquid levels in bottles and cans, was supposed to be in storage encased in a stainless steel box. A search of the site and among scrap merchants and waste facilities used by Irish Bonding failed to locate the source. It is presumed to have been landfilled.
In mitigation, the company said that staffing changes were responsible for the lack of monitoring of the source. It has since spent £500,000 upgrading its facilities to prevent a recurrence.