I've setup a website in IIS7 that gets its content from a fileshare using the UNC as the home directory, this site is running across 3 web servers. Now I would like the iusr anonymous account to have full control over this share. Should I create a domain account and change authentication to use that account on all the servers? What's the difference between application pool identity and specific user account? I would assume I would set it to use a specific user account with the one I created in the domain.
A general rule for all my web applications is to create a service account and have that application run under that service account. I would suggest you do the same for your situation.
Create the service account in the domain that you need. Then have your application impersonate as that account. Then if the account needs any permissions to folders, databases, etc., you can just give permissions to that account.