San Ildefonso, officially the Municipality of San Ildefonso, is a municipality in the province of Ilocos Sur, Philippines. According to the 2020 cens…San Ildefonso, officially the Municipality of San Ildefonso, is a municipality in the province of Ilocos Sur, Philippines. According to the 2020 census, it has a population of 8,190 people. The town got its name from Saint Ildephonsus.