The Irish Examiner reports that John died in hospital on Sunday, after having been ill “for some time”.
Her spokesperson said: "The family is very grateful for the support and kindness they have received but would kindly request that they are left to grieve in private."
John was raised on a farm in Ireland, where his parents still live, and began dating the ‘Downton Abbey’ actress two years ago.
The couple refrained from publicly acknowledging their relationship for around a year, until Michelle admitted that she was dating someone from Ireland.
After reports began to circulate last year, suggesting that the pair were engaged, she said: “I have a wonderful man in my life from Ireland. We were introduced by Allen Leech. That’s all you need to know.”
A private funeral is being held on Wednesday 16 December, in Cork, according to reports.
Michelle is best known for her role as Lady Mary in the internationally popular series, ‘Downton Abbey’.
Although the final series is now complete, one final Christmas special is due to air over the festive period.
Next year, Michelle is slated to star opposite Jim Broadbent and Charlotte Rampling in ‘The Sense Of An Ending’ and in the lead role of Vadim Jean’s ‘Consider Yourself’.