The 20-year-old has impressed in his four appearances for Gareth Ainsworth's side since joining the Chairboys on New Year's Day.
"I'm very happy to be staying for the extra few months," Harriman told the club's official website.
"I'm playing regularly and really enjoying it. Playing for Gaz is great and the lads have been really welcoming - they're a great bunch."
Harriman has also been named on standby for the Republic of Ireland Under-21s' friendly against Holland next Wednesday.