The irony in Bibot expecting to catch the Scarlet Pimpernel is simply that the Scarlet pimpernel then escapes right in front of him! Bibot reflects on the fact that no other guard has been able to spot him, but he will--just as the pimpernel, disguised as a hag and claiming to have the plague (this would have been horrifying at the time and would have chased many people away), sneaks by. This is an important point in the novel, however, because it sets the reader up expecting the Pimpernel to escape from situations where he encounters confident, prideful characters.
The irony in the expectations that Bibot has of catching The Scarlet Pimpernel is that despite his intense efforts and close eye, the Scarlet Pimpernel escapes, right underneath his very nose. Bibot prides himself on his vigilant capture of any escapees; he is confident that the elusive and enigmatic Pimpernel will never escape his clutches. The irony is that even as he is bragging about this, the man himself slides right on by, without Bibot even suspecting. The Scarlet Pimpernel did have an ingenius disguise; he was dressed as an old "hag," or woman, with a bundle of diseased joy in the back of her cart.
So, the joke is on Bibot; his ego is taken down a notch, and the Pimpernel wins yet another victory on Bibot's watch. I hope that helped; good luck!