I need to drill a hole through from my living room to my garage to put an ethernet run between them. I can only drill through from the living room in one spot due to the positioning of the electrical box that I want the ethernet cable to emerge at. I have measured where the hole will come out and found it is directly behind my boiler (the boiler is around 2cm from the wall and the wall is brick and plaster). Is just drilling slowly the trick here or is there another method someone can recommend to stop the drill bit from punching a hole in the boiler once it clears the wall? I was thinking of maybe placing something in the gap betweeen the boiler and the wall as a precaution to slow the drill bit down and give me time to stop drilling?
Don't. You'll ruin the cable.
As Nelson points out in the comments, CAT5 cables have a turning radius of 1 inch to avoid damage to the cable. In a scenario where the boiler is only 2cm from the wall, achieving this bending radius will be difficult, if not outright impossible. Additionally, the heat from the boiler is also likely to damage the cable if it's held at such close confines to it.
As such, it's highly likely that going through with this wiring scheme will damage the cable, so you should reconsider your plans and possibly drill the hole through the wall somewhere else.