The attorney general's office in Guanajuato, the violence-plagued state where Irapuato is located, said that 20 others were hospitalised with gunshot wounds.
Mexican President Claudia Sheinbaum said earlier on Wednesday that the victims included children, although the attorney general's office later confirmed only one casualty was a minor, aged 17.
"It is very unfortunate what happened. An investigation is under way," Sheinbaum said.
Local media reported the shooting happened during an evening party celebrating a Catholic holiday, the Nativity of John the Baptist.
The party was held outdoors and was being broadcast live on Facebook when the attackers arrived and opened fire, according to the newspaper AM.
A video circulating on social media showed people dancing in the patio of a housing complex while a band played in the background, before gunfire erupted.
Reuters was not immediately able to verify the video.
Guanajuato has been for many years one of the most violent regions in the country, where criminal groups fight over routes to price drugs and commit other crimes.
On Tuesday, five other people were killed in other parts of the state, according to the attorney general's office.
with DPA
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