Since you've got a shapefile for the populated areas, I'm going to go ahead and assume that is the polygon file and that you're using ArcGIS. The easiest option would be to use the Intersect tool.
Firstly, add your point text file:
- Click File > Add Data > Add XY Data
- Browse to your text file and select it
- Specify the X and Y fields, as well as the coordinate system, then click OK
- Right-click the new layer in the Table of Contents, select Data > Export Data
- Name the output feature class and click OK. This will give you data with OIDs, which are required for the Intersect geoprocessing tool. Add the data to the display when prompted.
Add your polygon shapefile via your usual method - drag and drop etc.
Open Arc Toolbox. Browse to Analysis Tools > Overlay and double-click the Intersect tool.
- Select your XY data from the Input Features box, then select your polygon data from the Input Features box.
- Name your output features
- Accept the other defaults and click OK
When the Intersect tool is finished, your new feature class will be in the Table of Contents. Open the attribute table and scroll to the end of the data. You will now have the attributes of your polygon data appended to the end of the table
Since I posted this answer the software has been identified as QGIS, so these steps won't resolve your issue. However, I will leave the answer here since others with a similar issue, but using ArcGIS, may find this question through search engine queries.