The 26-year-old centre-back scored the only goal of the game in Ireland's friendly win over Switzerland on Friday night.
Clark, who has made 16 appearances for Ireland since earning his international debut against Wales in February 2011, scored the only goal of the game at the Aviva Stadium, heading home Aidan McGeady's corner inside the opening three minutes.
The 26-year-old subsequently defended resolutely alongside centre-back partner Shane Duffy over the rest of the night as the hosts managed to limit their fellow Euro 2016 participants to just three shots on target.
O’Neill previously managed Clark during his four-year reign as Villa boss, when he handed him his first-team bow at the start of the 2009-10 campaign, and he hinted after the Switzerland game that the left-footed stopper is now in with an excellent chance of starting at the Euros this summer.
Speaking to Goal.com, O’Neill said: "Clark I’ve known since my Aston Villa days, [he] was a young lad, captain of the youth team at that time coming through, and he has come on very strongly.
"He played brilliantly in the last two matches of the qualification group for us and he’s carried on to tonight and his partner beside him young Duffy did very, very well indeed.
"Clark doesn’t give me any headaches what so ever," he added. "I’m absolutely delighted with him, I know his capabilities inside out. He’s come on strongly here regardless of what happens at club level and that’s very encouraging.”
Clark has features 155 times for Villa across all competitions since making his Premier League debut in August 2009, but his claret and blues career has been full of ups and downs so far.
The Harrow native was arguably the club’s rightful player of the year last season over Fabian Delph after producing some sterling performances to help them stave off relegation, but he has struggled to reproduce the same form this term and fallen down the pecking order as a result.
Nevertheless, with Villa expected to have a clearout over the summer now relegation looks all but inevitable, the academy product is an obvious candidate to stay on and help the the team try to bounce back to the top-flight at the first time of asking.