Did you specify a date post Sept. 19th to Eir? If so I'd be contacting them asking why they switched early.
If you didn't specify a date then Pure's terms are quite clear and they are entitled to charge you that. Their current contract includes a €150 charge is you cancel prior to the end of the contracted term.
For anyone else considering switching, you're always best to wait until you are outside the contracted period to initiate a switch as the telcos are notoriously poor at acting on instructions.