Who has been described in the above lines? Where is this person at this point of time? Why have his eyes been described as the eyes of a man who is used to looking at death?

The sniper is being described in these lines. The sniper is in position on a rooftop. From the text, we can infer that death has become a regular part of his life..... there is nothing shocking or uncomfortable about the act of killing. It is simply what he does.

On a rooftop near O'Connell Bridge, a Republican sniper lay watching. Beside him lay his rifle and over his shoulders was slung a pair of field glasses. His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.