John passed away on Sunday at a hospital specializing in hospice care in his hometown of Cork, Ireland. A spokesman for Michelle told the Telegraph: "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.”
Michelle began dating John more than two years ago after being introduced by mutual friend and Downton Abbey co-star Allen Leech. The couple kept their relationship out of the spotlight, but there were reports of an engagement earlier in the year in the British press.
Though always discreet with their romance, the pair had been pictured together at numerous high profile events, including the Venice Film Festival, Wimbledon and polo's Cartier Queen's Cup.
John was raised in Ireland and worked at FTI Consulting in London. Funeral arrangements have not yet been confirmed.