Chapter 89 (spoken as prophecy, heard by Quirrell):
“He is here. The one who will tear apart the very stars in heaven. He is here. He is the end of the world.”
Chapter 109:
Merlin left written instructions that the Mirror did not need to be sealed away, despite it having certain powers that might normally cause one to worry. His exact wording was that, given how painstakingly the Mirror had been crafted to not destroy the world, it would be easier to destroy the world using a lump of cheese.
I would say this implies that Quirrell ought to have triggered a safeguard on the Mirror, without Dumbledore needing to have installed it.
Chapter 89 (spoken as prophecy, heard by Quirrell):
Chapter 109:
I would say this implies that Quirrell ought to have triggered a safeguard on the Mirror, without Dumbledore needing to have installed it.