The West Bromwich Albion winger's third strike in two games extends Ireland's unbeaten record in Group D to four while it also puts a severe dent in Austria's aspirations to reach the 2018 World Cup in Russia.
Shortly after the half-hour mark, Jonathan Walters sent a glancing header wide of the far post before the Republic of Ireland survived a big scare down the other end.
It was a major let-off for Ireland but they should have been ahead at the break through another chance for Walters, but he sent the ball over from six yards after sliding in to meet a Robbie Brady cross.
After the restart, it took just three minutes for Ireland to move ahead after McClean latched on to a pass from Wes Hoolahan before driving the ball through the goalkeeper's legs from 14 yards.
Ireland forced Austria back into their own final third for five minutes after the goal but the home side eventually improved and just after the hour mark, David Alaba curled two long-range shots marginally off target.
Austria were having the majority of possession but as the game entered the final 10 minutes, they were not creating anything that was proving a big worry for Randolph.
That changed during the final 10 minutes as Martin Harnik flicked a volley straight at the Irish stopper before Janko headed well wide of the target despite being found in space at the back post.
Aleksandar Dragovic also saw a long-range shot deflected into Randolph's hands before Janko spurned the best chance of the lot as he headed wide from inside the six-yard box with less than 60 seconds remaining in Vienna.