Location
San Ildefonso, one of the smallest municipalities of Ilocos Sur, is bounded on the north by Sto. Domingo, on the south by Bantay, on the east by Bantay and the Cordillera Mountains and on the west by the West Philippine Sea.
Topography
The western part of the municipality is where one finds the coastal plains. The Bantaoay River and its tributaries flow through the town. 260 Ilocos Sur: An Almanac
Government
A fifth -class municipality, San Ildefonso is subdivided into 15 barangays. These are the following: 1. Arnap 9. Iboy 2. Bahet 10. Otol-Patac 3. Belen 11. Poblacion East 4. Bungro 12. Poblacion West 5. Busiing Sur 13. Quinamantirisan 6. Busiing Norte 14. Sagneb 7. Dongalo 15. Sagsagat 8. Gongogong