19th June 2018, 12:38 | Leeds
The 26-year-old prop, an imposing figure at 6ft 5in and 18st, gave up eating meat three years ago and now avoids all animal products while demonstrating that he can still perform at the highest level.
The former Widnes Vikings and Huddersfield Giants player, who was a key man for Ireland in the 2017 World Cup, was an ambassador for Veganuary at the start of the year as he sought to raise awareness and break misconceptions.
He said: "I read up about physiology and anatomy and, the more I looked into it, the more I personally came to the decision we are not designed to eat meat.
"It was gradual for me because of the nature of my career, I didn't want it to have a negative effect because, if my performance drops off, it affects my livelihood.
"So I took it step by step, stopped eating red meat, chicken and fish while each time researching more and obviously I was feeling good.
"I've not eaten meat for three years now and when I look back, I can't believe I ever ate it."
Mullally missed only two matches in the Rhinos' Grand Final-winning season in 2017 but it has been a stop-start campaign for him so far this year.
He dislocated a finger on the eve of the World Club Challenge in Melbourne, sat out a couple of matches with concussion and recently served two suspensions.
"I'm trying to get back to where I was last year consistency-wise," said Mullally. "It's a long year and I know it's going to fall into place soon."
More immediately, Mullally will be aiming to help injury-hit Leeds halt a five-match losing run in Betfred Super League when they host Catalans Dragons in a re-arranged game on Wednesday.
🇫🇷 Steve McNamara a dévoilé le groupe pour affronter Leeds en match en retard mercredi soir au Emerald Headingley Stadiumhttps://t.co/kBqbBZ7u96— Dragons Catalans (@DragonsOfficiel) June 18, 2018
🇬🇧 Steve McNamara has named the squad to face @leedsrhinos on Wednesday at the Emerald Headingley Stadiumhttps://t.co/Gz65EFylk6 pic.twitter.com/KwiGwHJu14
The French club beat the champions 33-20 in Perpignan earlier this month and can close to within a point of them by completing the double.