UEFA have informed the FAI that there is "clear and overwhelming evidence" of suspicious betting patterns on Athlone Town's 3-1 defeat to Longford Town last week.
The FAI announced on Wednesday that it was commencing an investigation into Athlone Town after receiving a report from the European football authorities.
Longford are not under investigation.
The letter from UEFA said: "There is clear and overwhelming betting evidence that the course or result of this match was unduly influenced with a view to gaining corrupt betting profits.
"The betting evidence ultimately indicates that bettors held prior knowledge of Athlone Town suffering a miniumum two-goal defeat, in a match which contained at least two first-half goals and at least four in total."
Longford scored just before half-time and full-time, and the report highlights the fact that the odds of goals being scored did not rise closer to the end of the halves, as they naturally should have.
Six-figure sums are believed to have been wagered on the game on Asian betting markets, far exceeding the normal amounts wagered on an Irish First Division clash.
The latest news on the FAI's investigation into illegal betting patterns on games involving Athlone Town in the League of Ireland. pic.twitter.com/qIUPRd3jOE— RTÉ Sport (@RTEsport) May 4, 2017
Francesco Baranca, general secretary of anti match-fixing Federbet, told the Irish Sun this week: "We were sure something was wrong according to our algorithm. The evidence there was more than enough.
"I’m talking about numbers, which are clear. We monitored the odds and it was crazy.
"I have no doubt that this match was fixed, according to those numbers.
"For the odds to change that much, it requires significant amounts of money to be placed."
FAI competitions director and integrity officer Fran Gavin and disciplinary regulations officer Rea Walsh will interview Athlone players next week, a number of whom have privately expressed shock at the revelations.
The club has stated that it "absolutely abhors match fixing and would never knowingly get involved in such unacceptable activity.
"The club will be fully co-operating with those conducting the investigation, and look forward to meeting with FAI officials next week.
"Should any of the allegations be upheld the club will take action against any wrongdoers."