Two goals from Billy McKay and a driven strike from captain Richie Foran gave the Highland side an unassailable advantage and ensured they would return to the top of the table after Celtic's win against Kilmarnock earlier in the day had briefly seen them take over.
The defeat was a first for the Edinburgh men since a 2-1 reverse at Kilmarnock on August 24, and their third of the season in the league.
"We've lost here for the first time in six games and to be fair we didn't deserve anything from the game," Fenlon said.
"They had more chances than us and took them and we gave away a sloppy first goal. In the second half they were the better side.
"The first goal was a poor mistake. It's hard enough coming here without gifting goals away to the opposition. It makes it more difficult.
"During the first half I didn't think there was much in the game but in the second half we just didn't turn up.
"We were trying to get subs on before the second and third goals but in the end it just wasn't to be."