The 41-year-old star secretly struck up a relationship with Garth months ago, reported Sun Online.
"They've been together for a few months but it's really ramped up recently. They get on brilliantly and she's a real hit with his family and friends. Idris is known as a ladies' man but he's really not at all. He'd rather have banter at the bar than spend all night grafting on women. Although he doesn't have to try anyway, women flock to him," a source said.
"Look, you probably think I'm out there all the time. But there's no way. They're all my fans. I miss the days when me and my boys could go to a barbecue, and go, 'Who's that shorty girl over there?'," he said.