The sort of irony at work in these two lines, spoken by Lennox early in the play, is known as prophetic irony. This means that the audience will not know the first time they watch the play that these lines are ironic—the understanding depends on a knowledge of what is going to come afterwards. Therefore, when Lennox says that Ross has a strange look in his eyes and that this suggests he is going to say something strange, it is not immediately obvious to the audience that there is irony in what Ross actually does say—namely, "God save the king."
However, if we know the play, we certainly know there is irony here. Ross is wishing good health for the reigning king, Duncan. In and of itself, this is ironic because Duncan will soon die at the hands of Macbeth, who he thought was loyal to him. Subsequently, the story Ross tells about the "traitor," the current thane of Cawdor, is also ironic because Ross is explaining how this man has behaved. Duncan, the king, then pronounces a sentence of execution on the thane of Cawdor and suggests that his title should be bestowed upon Macbeth—the clear implication being that Macbeth would be more deserving of the title because he would never betray his king. As we know, this turns out to be absolutely false.