The Austrian striker, 21, made his Aston Villa debut in 2010 - a year after Clark played his first game for the Villans.
And the pair have become close pals at Villa Park - but their friendship will be put on hold when they face each other for their national teams when the two countries meet at the Aviva Stadium.
The Ireland defender said: "Andi and I have had a chat about the game and I reckon we have a good enough squad to get a result against them.
"He is one of my good mates, one of the lads who has been at the club since he was 16.
"So we get on really well but if we're both playing on the day there won't be any love lost."