Don't inform them.
Provided it works fine on all browers and doesn't look a huge mess in IE8 (see 'Graceful Degradation') then why bother telling users to update their browser? The chances are if they're using an old browser it's because they don't have a choice - it could be a corporate network where they don't have permission / ability to update, for example. Telling them to update doesn't mean that they will. It'll more likely annoy them. "Everyone else is welcome, but you can't come in here with those shoes on sir".
You telling them that they are out of date isn't going to win you any favours from them. Ensure you've built the app using proper web standards so that it actually works. Telling them their browser is out of date is really just an admission on your part that "we haven't bothered making this site work properly on your device, it's your fault and you should do something about it". In fact it is not their fault the site doesn't work - it is yours. Especially considering this is an Intranet app, so you should have known what the browsers that it would be used on were so should build the app accordingly.
If you're not able to do anything about it then don't draw attention to the fact the site doesn't work ideally, just let them get on with it themselves. Ensure the site works and don't nag them that they're at fault.