The problem is that first you gotta define REAL last row. For instance, you can define last row in column X:
Code:
Dim lastRow As Long
lastRow = Range("X1").End(xlDown).Row
After that you can check your last cell in column Y:
Code:
Dim lastCell As Range
Set lastCell = Cells(lastRow, "Y")
And then you can check, whether this cell is empty, and if it is, then copy your A1 cell into that last cell in Y column:
Code:
If IsEmpty(lastCell) Then lastCell = Range("A1")
And that's all!