Thanks, Abby. 'The Merthyr Rising' by Gwyn A. Williams is a good source of information.

Also, there are two songs I know of:
'Merthyr Riots' by Rod Shearman, sung by Mabsant on the compilation album 'Valley Lights' (1986).
'Dic Penderyn' by John Stuart Williams and Geoff Cripps, sung by The Chartists (the authors' band) on 'It's Not Right' (1987).