The Blades found their opening goal a minute later after Chris Basham met a corner with a powerful header that Davies could not keep out.
The home side almost levelled just before half time after Adam Hamill whipped the ball across the box but Watkins could not connect to steer it home.
After the break, Barnsley were awarded a penalty after Hamill was brought down by Neill Collins, but Long managed to keep out Hourihane's spot kick.
Davies was required again for the home side as United went on the counter, and he managed to stop Sharp again from close range.
Winnall then came close to an equaliser after Hamill put in another good cross but the striker's header was kept out by the frame of the goal.
Hourihane would make up for his penalty miss in added time as he fired from long range beyond Long to earn the home side a point.
The draw keeps Barnsley in the relegation zone while Sheffield United remain in 12th in the League One table.