I've actually visited most of the schools in this race, so I knew who the announcers were talking aboot.
Look, there are only a small number of third level schools in all of Ireland. Athletics is not exactly the biggest sport in the country. These races are more comparable to a mid-level high school program, where you don't have enough runners to fill all of the slots, so people have to double up.
At my high school (in the US), the track team is over 300 kids. There is a huge tradition, however, that virtually every runner will also participate in the JV 4 x 400. I've seen runners who just finished the 3000 being told which 4 x 4 team they would be on. "You can be the anchor since you just finished the 3000."