Indeed, the client-side CellClickHandler client-side event is not fired when the ReadOnly property of the grid is set. I managed to find a workaround that may work in your case though. The idea is to create a div wrapper that hosts the grid and trap its onclick handler, then inspect the event arguments of the event to figure out the exact row. Rows have ids prefixed by the client side ID of the grid and ending with the index of the row. Here is my approach:

    <div id="divPlaceholder">  
    
    <igtbl:UltraWebGrid ID="UltraWebGrid1" runat="server"

    </igtbl:UltraWebGrid>
   
    </div>
   
    <script type="text/javascript">
   
    document.getElementById("divPlaceholder").onclick = handleClick;
   
    function handleClick(e)
    {
      
       
        var sourceElement = (e.srcElement) ? e.srcElement : e.target;
       
        if (sourceElement.tagName == "TD")
        {          
            var parentRow = sourceElement.parentNode;           
            var lastIndex = parentRow.id.lastIndexOf('_');           
            var id = parentRow.id.substring(lastIndex + 1);
           
            alert(id);
        }
       
    }
   
    </script>

Hope this helps.