While little Croia looked adorable in her navy uniform with a name tag attached to her chest.

Captioning his post, Conor penned: ‘The proudest Daddy in the whole world, my daughter Croía in big school today! I love you my baby girl, Princess of the World.’

That’s my girl: Conor McGregor shared his joy at his daughter Croia Mairead, four, starting her first day of ‘big school’ on Wednesday

His post was met with a plethora of supportive messages, while several of his fans joked that Croia won’t be messed with at school with Conor for a dad.

Conor also shares sons Conor Jr, six and two-year-old Rian with fiancée Dee Devlin, with the fighter confirming in July that Dee was expecting their fourth child.

It comes after Conor showed off his toned physique as he posed shirtless for a mirror selfie on Instagram earlier this month.

The sportsman displayed his heavily tattooed chest and arms while posing in a pair of blue striped shorts in a dressing room.

Despite not having fought in the UFC for two years, he hinted that he is raring to get in the ring again as he penned: ‘Fight ready.’

Fans have been eagerly awaiting news of an official clash between Conor and Michael Chandler, with Conor recently teasing at a December date for the clash.

He told ESPN: ‘You know I’m ready, I’m always staying in shape. If it could happen by the end of the year, that would be absolutely phenomenal. You know I’m ready tomorrow.’

The previous weekend, Conor also made a cameo appearance at Anthony Joshua’s clash, sending fans wild as he offered him his pint.

Sweet: The MMA fighter, 35, swapped the ring for a classroom as he took to his Instagram to share a picture of him cuddling his only girl as she sat at her desk ready for her big day

Proud: Captioning his post, Conor penned: ‘The proudest Daddy in the whole world, my daughter Croía in big school today! I love you my baby girl, Princess of the World’

‘Fight ready’: It comes after Conor showed off his toned physique as he posed shirtless for a mirror selfie on Instagram earlier this month

The moment came after Anthony defeated Robert Helenius, who stepped in at short notice as a replacement for Dillian Whyte after a failed drugs test, by seventh round knockout.

Conor was seen in the ring after the fight, and held his pint to a dazed-looking Anthony’s lips.

The boxer gladly took a sip, but then ignored Conor as he turned and walked out of the ring.

He later replied to embrace Robert, who was given oxygen by medics after a chastening fight.