The popular Skerries man came off at Braeside at the end of lap two on his Yamaha.
Sweeney, who qualified on the front row, was lying in third place when he went down, immediately bringing out the red flags.
The frightening incident shocked spectators but fortunately Sweeney was soon up on his feet and although feeling battered and bruised, he later finished third in the Open Superbike race on his MJR BMW before ruling himself out for the rest of the day.
Tweed, who was behind Paul Jordan and Sweeney when the crash occurred, said the incident ‘looked bad’ after he was forced to take evasive action to avoid the Republic of Ireland rider’s wayward Yamaha.
“It was a very nasty crash with ‘Micko’,” said Tweed.
“I was behind him when it happened and luckily just managed to miss his bike in the middle of the road. It sort of looked as though he might have lost the back-end of it, but I’m not completely sure.
“Thankfully he was okay though and he was soon back in the paddock afterwards.”
Sweeney had qualified on pole for the Superbike races but was unable to make the most of the opportunity for a victory in the feature class at Orritor following his crash.
Cork’s Mike Browne won the Open Superbike race from Adam McLean, with Sweeney riding through the pain barrier to take a battling third.
The headline Cookstown 100 Superbike race was run over four laps in the wet, with England’s Forest Dunn claiming a narrow win by half-a-second from McAdoo Racing’s Tweed.
Most of the fancied frontrunners did not take part in the race due to the greasy conditions and fading light.