Barangay Casilagan Sur

Situated in Banayoyo, Ilocos Sur, Barangay Casilagan Sur is one of 14 barangays that make up the municipality within Ilocos Region.

According to the 2020 Philippine census, Barangay Casilagan Sur had a population of 439. This made it the 8th largest of 14 barangays in Banayoyo by population, placing it in the middle tier of the municipality. Residents of Barangay Casilagan Sur accounted for approximately 5.54% of the total population of Banayoyo, which stood at 7,931 in the same census year.

Between the 2015 and 2020 censuses, the population of Barangay Casilagan Sur changed from 429 to 439, a shift of +2.3% over the five-year period. This places the barangay among those with growing populations in Banayoyo. Barangay Casilagan Sur is officially classified as rural, a designation applied to barangays with lower population density and predominantly non-built-up land use.

Banayoyo is a municipality comprising 14 barangays, and serves as the governing unit directly responsible for local services in Barangay Casilagan Sur, including public health, sanitation, peace and order, and civil registration. Within the wider province of Ilocos Sur, which contains 768 barangays across its component cities and municipalities, Barangay Casilagan Sur ranks 626th by 2020 population. Ilocos Sur is classified as a 1st by the Bureau of Local Government Finance, a category that reflects the province's annual regular income.

The barangay's legislative and executive affairs are handled by a Punong Barangay and the Sangguniang Barangay, elected nationwide during the Barangay and SK Elections. The next BSKE is scheduled for Monday, November 2, 2026, following the synchronized election schedule set by COMELEC Resolution No. 11191.