Prosecutors also took a tenth child, a 9-month old boy, into custody, the Jalisco state prosecutor's office said in a statement.
It said the three women were detained at a ranch in Tonala, a suburb of Guadalajara, while taking care of the baby boy. They told authorities they had been hired as nannies to take care of children in the process of being adopted, prosecutors said.
Police have raided several ranches to look for more evidence in the case, investigators said.
Authorities last week detained four women and seized nine children, all of them between two months and two years of age.
Prosecutors first opened the investigation last week following the arrest of a 21-year-old woman who was accused by her sister-in-law of trying to sell one of her children and of "renting" the other one.
The woman led authorities to three other women, all in their early 30s, who took part in the "renting" of the children, and seized nine children, including the 21-year-old woman's two kids. The other seven were seized from the Irish couples.
The first woman arrested claimed she had signed a contract with a law firm to allow her child to be photographed in different places in Jalisco state for advertising purposes. She told investigators that her child was taken for 15 days and that she received $500 pesos ($36 dollars) per day as payment, prosecutors said.
The other three women reportedly took the child and several others to a hotel in Guadalajara where they met with the Irish couples who believed they were going to adopt them. The couples then took the children to the nearby town of Ajijic, a lakeside resort popular with American and Canadian retirees, where they were staying while the adoptions were finished.
Officials are investigating whether the Irish couples and Mexican mothers were being tricked by the smuggling ring.
The Irish Embassy in Mexico said in a statement that it's providing consular advice to the couple involved and that the Irish council had traveled to Guadalajara to meet with the Irish citizens and local Mexican authorities.
Police are also looking for at least two lawyers with the Guadalajara law firm Lopez Lopez Asociados who were allegedly processing the adoptions in neighboring Colima state. The lawyers purportedly advertised in local newspaper for expectant mothers who wanted to give their children up for adoption.
Investigator said the foreign couples had been giving $1,200 pesos ($188 dollars) per week to the mothers since pregnancy, and paying for their medical attention.