Rt Revd William Paul Colton, wrote on Facebook: "What sort of twisted view of God motivates someone who claims to be a Christian to write to a cancer sufferer and suggest that her cancer is the will of God because she supports marriage equality?"
The letter to Una Mullally, from a 'No' voter in the upcoming gay marriage referendum in Ireland on Friday, wrote: "I was so sorry to hear about your cancer but maybe it is the will of God.
"After all you have been relentlessly pushing the twisted idea of gay marriage which would destroy the family as we know it and ruin the lives of generations of innocent children victimised by the narcissism of their 'parents'."
Ms Mullally, 32, who has Stage 3 cancer, which is not terminal, has been one of the most high-profile campaigners on the Yes campaign.
She tweeted a picture of the letter she received, but refused to reveal the name of the person who sent it, tweeting: "I would never put that online as it would be unfair. Rise above, innit."
Bishop Paul Colton is the most high-profile Church of Ireland bishop to openly support the campaign for gay marriage.
The Church has not taken a public stand on the referendum, but is encouraging people to vote "according to their conscience".
Roman Catholic bishops have urged their congregations to be guided by church teaching on Friday, by voting No.
However, some renegade Catholic priests have spoken out in favour of a Yes vote. In January, a Dublin priest announced during a Saturday night mass that he supported the referendum and that he is gay. His congregants gave him a standing ovation.
Irish voters are set to back the introduction of marriage equality by a margin of as much as two-to-one, according to the latest polls, making Ireland the first nation to legalise same-sex marriage by popular vote.
