Solar radiation is an important parameter in the study of electricity and/or thermal system installation. Direct monthly irradiance data measurements of the earth’s horizontal surface irradiance for the year 1985 to 2019 for nine stations (Sokoto, Birnin Kebbi, Maiduguri, Ilorin, Calabar, Port-Harcourt, Enugu, Iwo, and Ikeja) were collected from the achieve of HelioClim satellite website. The stationarity of the series was determined using the time plot of the irradiance data between the periods under study but the formal test of stationarity was carried out using the Augmented Dickey-Fuller (ADF) test with a p-value less than 0.05 signifying stationarity. Different autoregressive integrated moving average (ARIMA) models were fitted to the irradiance data for each of the selected stations. The results revealed that Sokoto, Maiduguri, and Kebbi have their maximum solar irradiance at about and their minimum at about , while Ilorin has its maximum solar irradiance at about and its minimum solar irradiance at about . Ikeja, Iwo and Port-Harcourt, Calabar and Enugu have their maximum solar irradiance and their minimum solar irradiance at about . It is therefore concluded that for Kebbi, Iwo and Maiduguri, the best model was ARIMA (3,0,3), for Calabar and Sokoto, it was ARIMA (2,0,2) while for other locations like Enugu, Ikeja, Ilorin and Port Harcourt, ARIMA (2,0,3) was the best model for forecasting irradiance in these study areas. The forecasted values of irradiance between January till December 2020 with its corresponding 95% confidence levels indicate good prediction of solar irradiance for future occurrence.