The move is "ridiculous" and highlights the need for blasphemy laws across the world to be scrapped, the BHA said in a statement released online.
BHA Chief Executive Andrew Copson commented: "It is ridiculous that Stephen Fry is being investigated for a possible breach of Ireland's blasphemy laws.
"Not only are such laws detrimental to free speech here in Europe, but they also enable countries like Pakistan, Saudi Arabia, and Iran to justify their own laws, which in the case of 13 states come with a death sentence."
Irish police are looking into whether Fry broke the Defamation Act during an interview with the Irish broadcaster RTE in 2015 when he was asked what he would say if he was confronted by God.
Don't put it off. Please, join us today & help us step up the fight against blasphemy laws and religious privilege: https://t.co/1ibdb9dMQy pic.twitter.com/3T4wRxj5SH
— British Humanists (@BHAhumanists) May 7, 2017
Speaking with presenter Gay Byrne, he said: "How dare you create a world in which there is such misery that is not our fault? It's not right. It's utterly, utterly evil.
"Why should I respect a capricious, mean-minded, stupid God who creates a world which is so full of injustice and pain?"
The police probe comes after a viewer reported Fry's comments were blasphemous.
Blasphemy is punishable by a fine of 25,000 euro under the Defamation Act in the Republic of Ireland; however the law has never been used.
Andrew Copson from the BHA added: "The current Irish Government is committed to holding a referendum on the abolition of Ireland's blasphemy law. It is past time that referendum takes place."
European countries take different approaches to blasphemy laws. Prosecutions have recently been made in Greece, Poland and Russia - while England and Wales, Norway, Iceland, Malta and France have abolished their blasphemy legislation.