Horwill was cited for allegedly stamping or trampling on lock Alun Wyn Jones in a ruck after the Lions referred the incident to match officials, but was cleared after a marathon hearing on Sunday.
The IRB on Thursday 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 part of "the process" and said it would distract neither him nor the Wallabies camp as the hosts head to Docklands needing a win to stave off defeat in the three-match series.
He also reiterated his defence in Sunday's hearing that he had "no idea" Wyn Jones was anywhere near his feet.
"I've played more than 130 professional rugby games and never been cited once and never been to any judicial hearings," he said.
"It was a complete accident, unfortunately accidents happen in rugby, it's a contact sport (but) there was no intent or malice."
Wyn Jones needed stitches around his eye, Lions coach Warren Gatland said after the Brisbane match.
Following Sunday's hearing, Lions assistant coach Graham Rowntree made his displeasure clear with Horwill's acquittal, but said the team would not appeal.
On Friday, a team spokesman said they had nothing to do with the IRB's action.
"The British and Irish Lions did not approach IRB about the appeal. That's on the record," the spokesman said.
The IRB's move enraged the Australian Rugby Union and added to a difficult week for the Wallabies, who have had to deal with a raft of backline injuries from the first test and 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. Horwill reiterated 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."
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- Alun Wyn Jones