Under Construction & block other pages in CI

i think the safest way, and its actually very fast --

www.mywebsite.com/index.php
replace the codeigniter index.php with your "under construction" message
this completely eliminates any chance of someone opening up
mywebsite.com/user/dashboard

make a folder like
www.mywebsite.com/developerpreview

and put in the codeigniter index.php
www.mywebsite.com/developerpreview/index.php
the index.php in that folder points to the application folder you are working on

when you are ready to go live, replace the top level under construction page with the codeigniter index.php
pointing to the appropriate folders