Irosin population has grown from 8,644 in 1903 to 56,662 in 2015 with a cumulative increase of 48,018 or 556%.  Within the span of 112 years the population rose at an annual average rate of 1.68% per year. The population has practically doubled from the 1970 population to the present 2015 population or 45 years. The growth rate for the census period 2010 and 2015 is 1.72%. At this present growth rate, the doubling time of population is 40 years.

The 2015 urban population is 14,389 or 25.47% of the total population while the rural population is 42,273 or 74.6% of the total population. In the 2007 NSO census, 2,688 households or 27% of the total households are in the urban area while 7,225 are in the rural barangays. The most populated barangay is the urban barangay of Bacolod with a population of 4,518 in 2015. The second and third most populous are the rural barangays of Monbon and Gabao with populations of 4,414 and 4,286 respectively. The urbanization level, or growth in the proportion of population living in urban barangays, has been steadily declining in the past 20 years. This declining trend is correlated to the disparity in the population growth rate of rural and urban barangays. The annual growth rate of urban barangays from the 1980 to 2015 ranged from -0.70% to 1.15%, while the rural barangay’s growth rate ranged from 1.41% to 2.98% in the same period.

The gross population density of the municipality is 3.75 persons per hectares in 2010. Urban density is 45.2 persons per hectare compared to the rural density of 2.86 persons per hectare. The highest density of population is found in Bacolod with 245.85 persons per hectare, followed by San Juan, San Julian, San Agustin and San Pedro. The built-up density of urban barangays is almost the same as its gross population density except for San Pedro which has a large tract of agricultural lands. For the rural barangays, the highest density is Gabao with a population density of 10.67 persons per hectare and followed by Gulang-gulang, Macawayan, Buenavista and Batang with densities of 9.16, 8.88, 6.45 and 5.59 respectively. The built-up density of rural barangays is significantly lower since habitation areas are usually clustered on small areas surrounded by vast agricultural lands.