see the first table on this page http://en.wikipedia.org/wiki/German_pronouns
'du' is informal second person singular (YOU) 'ihr' is informal second person plural (Y'ALL, OR YOUSE - when you're talking to more than one person)
Note 'ihr' also means 'her' " Ich gab es zu ihr" (I gave it to her)
Usually you can tell the which ihr it is by the main verb.
Ihr hat - she has Ihr habt - y'all ahve
Ihr esst - y'all eat Ihr isst = she eats