In the letter, the writer claims that her recent cancer diagnosis was the 'will of God' and was connected to her support for a yes vote in Friday's referendum.
Una recently wrote a well-received piece in the Irish Times detailing her diagnosis with cancer and her struggles to accept herself as a lesbian.
A vocal advocate of marriage equality, Una is no stranger to criticism and even says this letter isn't the worst she's ever received.
Many tweeted their disgust at the contents of the letter, saying that no matter which way you are voting on Friday, this was deplorable.
That letter sent to Una Mullaly is almost a parody in it's ridiculousness. Sadly, this person exists. I can't comprehend that much hate.— Órla (@orlabaz) May 19, 2015
@UnaMullally I'm so sorry that you have to put up with this horrible abuse. These should be published with names (if they gave them)— Ruth Hegarty (@EggandChick) May 19, 2015
Una has said she has no intention of making the name of the letter-writer public - the person signed the letter with 'Best Wishes'.
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