Portland Prison, [ which is now the Young Offenders Institution], not to be confused with the present Prison called the 'HMP Verne'. Any records should be at the National Archives [TNA]. I cannot find any records post 1890's, but it would be better to e.mail TNA and ask what is available. Was the person a local ? or from another part of the country ?
Local newspapers to his area of living might have referefences to the sentence/crime.
msv
Logged
Vigurs / Rodwell / Kite / Molcher-Malsher and any variants of the above.
Cornwall, Beds, Berks, Kent, Northants, Norfolk, London