um, maybe.
in isometric your original x and y coordinates are skewed and don't follow the x and y dimensions of the display. any given x-gridline (on the grid data structure) takes up several different y values on the screen (and vice versa for y-lines and x values). but the diagonals (on the data structure) map to constant x and y positions (on the display). so you can measure the distance in diagonals between any given point and the origin (on the display), and that'll tell you which cell you're addressing.
for each diagonal in one direction, the sum of the x and y adresses (on the data structure) for all cells on that line is constant. in the other direction, the difference between x and y is constant. so if you have both of those values, you've found the cell - but these are the same values you get from counting the x- and y-distance from the origin on the isometric display.
in the example, the cell you're after is one diagonal east and three diagonals south of the origin. so you find the cell in the target area where x-y=1 and x+y = 3, and that's your guy.