Leinster will now face the Bulls in a repeat of last year’s final in Dublin on June 19.
The Stormers trailed 13-11 against a tiring home side when Ackermann recklessly entered a ruck with a shoulder to the head in what was a clear act of foul play.
He initially received a yellow card, which was later correctly upgraded to red on bunker review.
Salmaan Moerat was more unfortunate to receive a yellow shortly afterwards for what was deemed cynical play, while Leolin Zas received a yellow for a deliberate knock-on in the first half
“I thought up until the (red) card we were in it,” Dobson said. “I thought we could see some cracks starting, maybe physically, in Leinster. I think a big, big, big moment was getting into their 22, but we didn’t often do that in the second half.
“If we’d converted there, I think we’d have had reason to believe we could put them under pressure because I could see them looking a little bit tired.
“I really apologise for the (red card). That’s where you have to get rid of that in rugby. Tucked shoulder to the head. You know, because then guys aren’t getting a jackal. My only thing to (Leinster coach) Leo (Cullen) was to say sorry for that.”
Had the Stormers been able to hold their discipline, they could well have come away with the win, which would have seen them host the final in Cape Town.
“I said to the guys at half-time that we won’t win this game if we don’t stay with 15 men on the field,” Dobson added.