PiBa-NL

Code to change a request from / to /app1/
  reqirep  ^([^\ :]*)\ /(.*) \1\ /app1/\2

If urls in the response contain absolute urls it might be required to use this:
  acl    no_redir url_beg   /app1/
  reqirep  ^([^\ :]*)\ /(.*) \1\ /app1/\2 if !no_redir

The code makes sure that the method and url-path behind the / stays the same. Which method you need exactly might depend on the application thats running.
For readability of the above how change a request from /app1/ to /app1/app1redir/
  reqirep  ^([^\ :]*)\ /app1/(.*) \1\ /app1/app1redir/\2

PiBa-NL

To send the ip addres of the client/webbrowser to the server/webserver behind it there are a few options:
 1- option forwardfor
 2- send-proxy
 3- source 0.0.0.0 usesrc clientip

1- option forwardfor
This is an easy option to configure in haproxy, it does require that http layer7 processing is used 'mode http' and the webserver/ webapplication that wants to log or use the ip of the client must use the http-header 'X-Forwarded-For' to read the clientip.

2- send-proxy / send-proxy-v2 / send-proxy-*
This is can be used both with mode tcp and http, it does however require that the server also understands the proxyprotocol. Some applications have added support for this protocol which adds a few bytes with ip information before the actual request.