What is the Defence Force's equivalent of an MOS?

We dont use an MOS system, so no special designation.

There is no system.

A technician or certain post holders are in receipt of technical pay and they might be called an aircraft technician or an engine room artificer,titles which define their role but the DF does not have a numbering system like the Americans. Our Military essentially copied the British system back when the National Army was being formed, as it was what everyone was used to, their having been so many Irish in the British military. Our rank structure would follow British systems in that we have similar ranks. The American numbering system is also to do with pay grades/specialist pay and the level of training required for a post. As an example, driving training is given to a much wider range of ranks in the US military than in the Defence Forces, because traditionally, Americans tend to have driving licenses much earlier than Irish people do, they have vastly more driving requirements in their huge military than we do and they give driving tuition much earlier in a military career than we do. It is entirely possible to spend an entire career in the DF and never have DF training to drive anything, even the smallest car.