It's for the Voyager Computerised Fleet Database System. All fleet numbers have to have four digits and they are using the same numbering system for the train and bus fleets. All the train numbers had 8000 added to them to avoid clashing with Flexibus and Ulsterbus numbers just as all Citybus fleet numbers had 2000 added to them in 1973 to avoid clashes with Ulsterbus numbers. Numbers in the 5000 range are for company cars and non-PSV vehicles. Maybe 8000 was chosen for trains because it also suited the Iarnrod Eireann system?