Chris, where do you get that 15 minute cover code from? That is new to me.
Is that an IRC thing?
No its an NEC thing...
Start at 334.10 Uses Permitted,
(1) One- and two-family dwellings. (does not apply, garage is not a dwelling unit)
we can skip (2) because that does not apply either,
so we go to (3)...
(3) Other structures permitted to be of Types III, IV, and V construction except as prohibited in 334.12. Cables shall be concealed within walls, floors, or ceilings that provide a thermal barrier of material that has at least a 15 minute finish rating as identified in listings of fire rated assemblies. ( This applies in the OP's case)
Check out the Definition of
Dwelling Unit as well... a garage does not meet the definition.