The jury in the case failed to reach a majority verdict on 12 other counts of indecent assault and rape, they told Mr Justice Paul Carney at the Central Criminal Court.
The accused man, now 62, had pleaded not guilty to all charges relating to alleged abuse between January 1965 and June 1975.
The man told gardaí that he “feelings” for the woman but they never had a sexual relationship.
He told gardaí his foster mother, who was the complainant’s guardian, “became aware of feelings between us and she wanted to interrupt this”.
He claimed he was 23 at the time and the complainant was 15 or 16.