The Guardian reported that both men have been through tough times, with Mr O'Sullivan becoming homeless and Mr Murphy suffering from giant cell arteritis, a condition that affected his optic nerve.
Under Irish law, O'Sullivan would have to pay 50,000 euros in inheritance tax on the house, which he would only be able to afford if he sold it.
Murphy, who is 83, and O'Sullivan, 58, chose to marry when they discovered how much in taxes they would have to pay if O'Sullivan inherited Murphy's home, as intended upon Murphy's death, according to the Guardian.
"I've known Matty for 30 years. He became friends with all my friends, they all loved him".
The pair have been best friends for nearly 30 years, but Matt had never married, while Michael is separated with two children. "I would go over and stay with him the odd time but never full time".
Mr Murphy and Mr O'Sullivan told local news that they intend to marry on December 22 if weather allows, and if not the wedding will be held in January.
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The father of two daughters and a son from previous relationships, added: "I love Matt... but not in a sexual way".
When they realised that no inheritance tax would be payable on the house if it were left to a spouse, the couple chose to tie the knot.
His family is on board, and asked whether he would tell women he will date about their marriage, he said: "Probably so, of course I have to tell her!"
He paid tribute to Ireland's LGBT (lesbian, gay, bisexual, transgender) community.
The couple got married in a former hospital on Dublin's Grand Canal Street, followed by a meal for five at the nearby Gasworks bar.
"What happened was, Matt said to me, "Look, I'm going to leave you the house", and I said it was a nice idea but because of tax reasons I'd have to pay half of that to the Government". It is Murphy's first marriage.