Mr Zelensky told a travelling Irish delegation he was once a fan of McGregor but said he changed his opinion due to the Dublin man posing with Russian president Vladimir Putin.
McGregor posted a photograph of himself and Putin at the 2018 World Cup final in Russia on Instagram four years ago.
He captioned the picture: "This man is one of the greatest leaders of our time and I was honoured to attend such a landmark event alongside him."
A video of the encounter shows the Crumlin fighter (33) warmly greeting the Russian leader, talking through a translator.
They then pose for a photograph, with McGregor putting his arm around Putin before one of the Russian leader's security team asks him to remove it.
After meeting Putin, McGregor revealed he gifted the Russian president the first bottle of his Proper 12 whiskey but was unsure if Putin actually received it as his security took it off him to be checked for poison.
"President Putin's security detail is second to none, as I'm sure you know, you don't mess around with Vladimir," he added.
McGregor's spokesperson said the fighter "often takes photographs with event hosts" and added that "this photograph was years ago".
Mr Zelensky held a 45-minute meeting with Ceann Comhairle Seán Ó Fearghaíl and Seanad Cathaoirleach Mark Daly last weekend in the Ukrainian capital.
It was the culmination of a state visit by the two politicians and their political staff during which they also held talks with Ukrainian prime minister Denys Shmyhal and speaker of the Ukrainian parliament Ruslan Stefanchuk.
Mr Zelensky discussed a wide range of issues with the Irish delegation but mostly focused on the support Ireland can give Ukraine during and after the war.
They also discussed Ukraine's bid to join the European Union, which Ireland has committed to support.
Mr Zelensky spoke about his recent meeting with U2 frontman Bono, who visited the war-torn region and performed in a subway station in Kyiv before their meeting.
But while he spoke glowingly about Bono, Mr Zelensky was less than impressed with McGregor.
"He felt he wasn't being supportive of Ukraine," Mr Daly told the Herald.
"This man is fighting a war but he knows what's going on all around the world, including in Ireland."
McGregor is among the world's highest paid athletes, earning €37.3m in the last year, according to Forbes.
His business ventures outside of MMA include his pub The Black Forge Inn in his native Crumlin, as well as launching his Proper 12 whiskey in 2018, although he has since sold his majority stake.
Mr Daly said that in their meeting with Mr Zelensky he and Mr Ó Fearghaíl committed to advocating for the EU to accept Ukraine's request to join the union.
"We will be asking our colleagues across Europe to support their application to join the European Union."
Mr Daly said Mr Zelensky is "really a leader of all times".
"When they are teaching leadership in universities for years to come, this will be the standard," he added.
The Ukrainian president also asked that Ireland sign up to a scheme that would see European countries adopt a region of Ukraine and help with rebuilding the country.
Mr Daly and Mr Ó Fearghaíl flew into Warsaw in Poland before taking a four-hour car journey to the Ukraine border.
They were then taken under heavily armed guard on an 11-hour train journey to Kyiv.