The 22-year-old, who joined the Scottish outfit from Swindon Town this summer, scored 18 goals in League One last season.
"We watched him a few times at Shrewsbury and then he got himself a real good move from Swindon and did really well. Fortunately we managed to take him from there and bring him to the club," Fenlon told the club's official website.
"He will be a big addition. Obviously he has a very good goalscoring record but he is a real centre-forward and that's something we have missed."
Collins had previously been linked with a number of Football League clubs.