Sean McCarron was on target for the Ollie Horgan managed side with a stunning long range effort in the first half.
But Shels fought back with Graham Couglan equalising from the penalty spot after Dylan Connolly went down in the box under pressure from Ciaran Coll.
Both sides could have won the game with chances at both ends late on.
Shels will be more disappointed as Longford are now the favourites to win the league.
Harps remain fifth but are eight points adrift of the promotion play-off spot.