The 27-year-old Donnelly gave the visitors a shock lead in the 13th minute when he volleyed in from close range for his first goal of the season after the Light Blues' defence had failed to deal with a corner.
The League Two side, cheered on by 1,100 fans, had to withstand inevitable pressure thereafter but were on course for an historic win until Mohsni levelled in the 77th minute when he knocked the ball and Rovers keeper Neil Parry over the line.
The Cliftonhill club will cash in on the replay which will take place at New Douglas Park next Monday with the winners facing Dundee United in the semi-final at Ibrox while Aberdeen take on St Johnstone in the other last-four game.
Donnelly is Rovers' longest-serving player, having joined in 2005, but he is struggling to shake off the "what if" feeling from the trip to Ibrox.
"The fans were great, they enjoyed it but the players are devastated to be honest," he said. "Not many teams will get the chance to go to Ibrox and get the win and I thought we thoroughly deserved a draw at least.
"So it does feel like a defeat, yes. I was absolutely delighted to score, it was just a pity we couldn't hold on for the win.
"But it is a cracking result for the club. Financially and for the fans, you seen what it meant to them. As a player, it is right up there, it rates really highly.
"But I thought we could have won the first game so we will try to win the replay."