ASP.NET Problem

What exactly is the external source? If you have control over it (the external source) you could get around it by firing an event as soon as you have inserted your data from the external source.

The external source will be a mobile application connected to a different server which inserts new data to the main (shared) database. The users which are logged on must then be informed that new data is present...preferably in real time :/. 

I would recommend that your client always has to go through your server and never directly to the database. To my knowledge also most systems which claim to be real time are not strictly real time, so depending on your application you can, most of the time, get away with 'old' data. Usually allowing 5-15 minute intervals suffices.

Another thing you could do would be to make the entire process event driven, so basically once a mobile device updates the database, through a process which I am assuming you have control on you can fire an event and the server will make the data available and send a notification (another event) to all the logged in clients. You could use multithreading to make this process a bit faster.

Laslty, I do not have much experience with web development in .NET, so maybe you could wait a day or two till people with more knowledge can get here and help. I also tend to find http://www.stackoverflow.com very helpful, so you might want to give it a shot.

The easiest way to do is using an ajax timer that triggers a method every couple of seconds. The timer is on the left hand side of visual studio  under the section ajax. drag it on the aspx page and double click on it to create the ontick event.

But that would be polling, which, as I understood, is to be avoided. Nevertheless, that is how I would have done it. Maybe as Xeon suggested, StackOverflow would have the answer. I rely on them too.
 

If you will rever to stackoverflow for design pointers (which I recommend) be sure to include what you have thought and/or tried. Keep in mind that these people will provide you with assistance out of their good will, and do not like questions like: I have this problem, please solve it for me...

Thanks for your feedback!! I ended up implementing an AJAX timer to poll the database as there doesn't seem to be  any alternative :/.