You don't have to code anything to get this to happen, you configure it. I have a log in page setup within the content tree with the log in webpart at /Global/Logon so:
Wherever you have your "global" log in page setup at modify your web.config < forms > node to use that URL. i.e.: Global/Logon
<forms loginUrl="Global/logon" defaultUrl="Default.aspx" name=".ASPXFORMSAUTH" timeout="30" slidingExpiration="true"/>
In your /Admin directory, modify the web.config to look something like this:
<allow roles="CMSBasicUsers, CMSCommunityAdmin, CMSDesigner, CMSDeskAdmin, CMSEditor, CMSReaders, MarketingManager, or whatever roles you need"/>
Then in your sites settings make sure you set your Website logon page URL property in Settings>Security & Membership to ~/Global/Logon.
This should handle what you need and auto redirect your users anytime they request the /Admin or any other secured section of your site. The only problem with this is if your site has errors or cannot compile, you cannot get to the Logon page because you are using a page in the content tree. I have it setup you can access it on the local server with a little configuration but that is only in the event of an emergency.