The ship had been checked out. No guns.
When he saw the O'Connell flag. he must have surmised that's how they (thought) they were going to beat him., i.e. transferring the cargo after the ship had been searched.
He already knew the O'Connels had a connection to the fund raising. Then there was the "ourselves alone" slogan.
He also knew the meaning of that "overseas" being related to the armed fighting group.