One of the last unsolved secret codes sent by the Irish Republican Army in the early 20th century has been cracked by a Brisbane data scientist.

The cryptogram, which eluded the world's top code-breakers for decades, revealed that explosives were stolen from the IRA's British rivals some time in the 1920s.

Gelignite was often used in IRA bombings. Here was an explosion in Belfast. Credit:AP

Using a columnar transposition cipher, a decoder that rearranges texts in columns, University of Queensland researcher Richard Bean turned 51 random letters into a clear message.

Gelignite Scotland states they raided and obtained some of this.