Six of the defendants are serving prison terms for other crimes.
There was a period between 1990 and 1992 that he served a number of short prison sentences in Ireland for various crimes.
Johnston was arrested in 2006 and served 90 days of jail time for various crimes.
The police said the two men met in the early 1990's when both were serving time in Oregon for sexual crimes involving children.
He later served three prison sentences for various crimes, including robbery and murder.
There are currently 73 Americans serving such sentences for crimes they committed at the age of 13 or 14.
Twelve of the defendants were already serving prison terms for other crimes.
He served several years in prison for other crimes.
All are already serving long prison terms for violent crimes.