Section 24, The Sexual Offences Act, 2012

(1) It is a defence to a charge under sections 19, 20, 21, 22and 23 if the defendant satisfies the court on the balance of probabilities that– (a) before the time of the act concerned, he or she had taken reasonable steps to find out whether the child concerned was of or over the age of 18 years; (b) at the time of the act concerned, he or she believed on reasonable grounds that the child was of or over the age of 18 years.