Horwill was cited for trampling on fellow lock Alun Wyn Jones after the Lions referred the incident to match officials, but was cleared after a marathon hearing on Sunday.
The IRB said it would appeal the verdict, effectively questioning the governing body's own appointed judicial officer.
Horwill is clear to play in Saturday's second test at Melbourne's Docklands Stadium but will have to face the music next week.
"I got a very fair hearing the first time and I expect it to be no different come the second time," Horwill told reporters in Melbourne on Friday.
"I don't know too much about it, I've been focused on the game and once the game's finished, obviously we'll have a better look at it."
The 28-year-old shrugged off the appeal as just "part of the process" and said it would neither distract him nor the Wallabies in general as the hosts head to Docklands needing to win to stave off defeat in the three-match series.
He also reiterated his defence in the first hearing that it was an accident and that he had "no idea" Wyn Jones was anywhere near his feet.
The IRB appeal enraged the Australian Rugby Union and added to a difficult week for the Wallabies, who have needed to deal with a raft of backline injuries from the first test amid a number of off-field distractions.
Winger Digby Ioane was ruled out of the series on Monday, but not before a warrant was issued for his arrest for failing to attend a Melbourne court on an assault charge.
The Wallabies were also embarrassed by a photograph of flyhalf James O'Connor and fullback Kurtley Beale taken at a fast food restaurant at 4 a.m. on Wednesday.
The Wallabies leadership group said they were too focused on the second test to sanction the pair, but punishment would be forthcoming after the match and Horwill re-iterated that it would be handled in-house.
"I think we'll deal with that in due process," he said. "They understand what's going on and they understand that they need to make sure they everything for the team.
"We'll deal with that on our own terms in our own private way."
