The alleged victims -- both probationary gardai aged in their 20s -- were said to be "distressed" when they made a complaint which was examined by a chief superintendent.
The chief superintendent then appointed an inspector from outside the area where the alleged assaults happened who investigated the matter.
A detective inspector was then appointed to deal with the case. The detective inspector interviewed several witnesses.
In late June, a file on the case was sent to the DPP, who decided late last month that the case should be prosecuted at District Court level.
The senior garda at the centre of the allegations has been a serving officer for over 20 years and has previously escaped serious sanction over a separate disciplinary matter.
It is understood that the male garda -- who is aged in his 40s -- has not been suspended from his duties but has been confined to desk duties while a major investigation was carried out into the alleged sexual assault incidents.
Sources say the garda has been transferred from the station where the sexual assaults are alleged to have taken place to another south Dublin station.
The officer is due before Dublin District Court within the next fortnight to be formally charged with the offences.
The officer -- who is considered a "steady investigator" -- has been praised in the past for his investigative role in a dangerous driving causing death case, which sources said he handled in a very sensitive way.
The allegations against the officer have sent shockwaves through the garda station.