Insulating a garage – from the outside or the inside

Condensation is definitely a concern in colder weather. If you create a thermal break that isn't properly sealed against air leakage, warm air from the house will seep behind the walls, bringing lots and lots of moisture with it. Meanwhile the bricks, which are now insulated, are going to be very very cold, and when warm air gets cold it lets go of its moisture - and thus we get condensation.

That condensation would then get trapped inside the walls - and long before you have a problem with brick deterioration you're going to have mold issues that lead to health issues.

The best thing to do to prevent this is to insulate with a substance that also serves as a vapor barrier, which will prevent any air (and thus any moisture) from leaking into the closed cavity. The best product for this IMO is spray foam insulation, because it fills in all the nooks and crannies and IS the vapor barrier.

Alternatively, you could use fiberglass and cover it over with plastic to form a vapor barrier, and this can work IF you properly seal it all the way around the edges, seams and any holes that occur.

Either approach, however, is going to require you to sacrifice living space, as for a proper thermal break you're going to probably want at least 4" of insulation.

Now - with regard to insulating your spare rooms. Don't insulate your interior walls. Put proper insulation in the external walls of those rooms, so that they don't lose heat to the outside. Yes you will be heating them, but if they're properly insulated they're not going to cost much to heat and there are numerous other concerns with turning an inside room into a "cold" room - it's a finished space, after all, and you don't want to risk damaging that by letting it be freezing cold in there.

Insulation slows the loss of heat, it will not raise the temp of the wall above the dew point unless there is a source of heat inside. (Heater, leakage from another source, or a day/night average temp above the dewpoint.

You may be best looking for a way to lower the humidity in the space. Removing standing or leaking water, finding vapor leakage from the conditioned space or other sources of moisture (clothes dryer vented into the space). You may want to contact a basement specialist contractor. Concrete is porous, and if the wall were not sealed with a watertight membrane before backfilling moisture can be leaking through the walls. If you do not wish to heat the space, adding ventilation to get the wet air out will help.

Once that is done you can think about insulating. The easiest, though expensive, solution is to spray cold cell polyurethane spray foam on the wall. This will act as a vapor barrier and insulation, and it has no air space behind it for condensation. Second best is solid foam board insulation outside, from top to bottom of the wall (even underground).