Hi Nicola,
It appears that, besides the successful escape of the Hougoumont Fenians, there were two Hougoumont convicts who escaped and were recaptured in March, 1868. They were each sentenced to a further three years, six months in irons.
The convicts were not named in the newspaper report, but perhaps one of them was your Richard, or a similar attempted escape may have been the reason for his extended sentence?
"ESCAPE OF TWO CONVICTS BY THE HOUGOUMONT AND THEIR CAPTURE BY THE WILD WAVE"
The Inquirer & Commercial News, Wednesday 1 April 1868. (Source: TROVE)
Cheers, Anne