gtk:text-view-iter-at-location (view x y)

view -- a gtk:text-view widget
x -- an integer with the x position, in text buffer coordinates
y -- an integer with the y position, in text buffer coordinates

The gtk:text-iter iterator at text buffer coordinates.

Retrieves the iterator at text buffer coordinates x and y. Text buffer coordinates are coordinates for the entire text buffer, not just the currently displayed portion. If you have coordinates from an event, you have to convert those to text buffer coordinates with the gtk:text-view-window-to-buffer-coords function.