120 years ago is about the time they started doing double brick walls. Leaving air between the inner and outer brick walls was basically the first kind of insulation.

In the 1970s they figured out to put polystyrene beads in the space between the walls. This idea has recently been improved by better materials, so it's still possible to update the insulation on these old houses. Whether it's financially feasible is a different question.

Another method is to put more insulation on the inside, but this takes up space and early attempts in the 1950-1980s proved to do very little except creating a fire hazard. If a house has those old panels on the walls, it's probably better to remove them.

A neighbour of mine recently put very thick insulation on the outside of the house. Must have cost a fortune and it looks weird, but I guess it works really well. It also requires that all windows are moved, so it's probably easier to just build a new house.

So there a few options, but money is likely better spend on changing the roof, windows or the heat source.