Gerry was a member of the IRA when he was a teenager
just one of the things Jim Sheridan left out.
No, you're thinking of Paul Hill. Gerry Conlon was a petty thief, & the IRA doesn't recruit criminals.
Who are you? Who? Who? Who? Who?

Gerry was a member of the IRA when he was a teenager
just one of the things Jim Sheridan left out

Joe (blows) considered Gerry as a patsy for the IRA cause, since Gerry was a common thief that DREW attention to the whole local IRA group in the neighborhood, so this may have been a way to get rid of Gerry the local, "their always robbing our houses" thief, only when a whole bunch of other people including a 14 year old kid got involved in the mix, then and ONLY then, the IRA thought about about doing the right thing otherwise Gerry would still be robbing brownies at the jail for dinner. Oyyyyy!
Spoiler alert for them spoil sports out there! Y'all like spoiled milk, stop crying over it!

The only thing I remember is Joe telling Gerry not to sit next to him during the movie, but I just took that as Joe not wanting Gerry for this plan of his.

I don't recall Joe keeping his distance before the movie.
Awe Skinny you got blood all over my trousers! Jeez I'm real sorry sorry Frank