Yes, I know this title sounds like a contradiction. Yet the irregular opponent operates in tiers.

At the “bottom” tier of organization, as per Training Circular 7-100.3, Irregular Opposing Forces (source of diagram), there is what that document calls “insurgents”, ones devoted purely to doing damage.

Next are what it calls “Guerillas”. The definition is “

“A guerrilla force is a group of irregular, predominantly indigenous personnel organized along military lines to conduct military and paramilitary operations in enemy-held, hostile, or denied territory (JP 3-05). Thus, guerrilla units are an irregular force, but structured similar to regular military forces. They resemble military forces in their command and control (C2) and can use military-like tactics and techniques.”

(Bolding added by me)

The document holds “guerillas” to be more organized and more capable of conventional-ish action than “insurgents.” It lists (obviously rough) organizations up to brigade size.

Then it gets trickier. Then there emerges “regular forces” that are intended to fight and hold ground conventionally. The Vietnam-era “Handbook on Aggressor Insurgent War” (FM 30-104, 1967) has a sample regiment of these regular forces organized as follows.

FM 30-104 rightly notes that these are organized similar to conventional Aggressor rifle regiments, only with lighter equipment. This flows right into the highest tier, consisting of…

• Forces trained and equipped similarly to their external patrons (since very few unconventional forces can grow this powerful without outside backing). These are less interesting from an organizational standpoint, as the only things really distinguishing them are the origins of their forces and sometimes skill.
• Irregular forces that have the size and equipment to succeed at conventional operations. These will have de facto infantry, motor vehicles (the infamous “technicals” ) and a smattering of supplied/captured AFVs, operable in what would be considered “detachments” in more structured armies in terms of their size and (lack of) organization.