An informer, on the police's payroll. Derived from the fact that payment comes in the form of a giro cheque, and that the police in England are sometimes called 'the blues'.
Yo chill out man, did you hear about Bobby? He's an informer, he's a blue giro man, chill out yeah!
Payment made to a informant or grass by the pigs.
I reckon he's on the blue giro after the police let him off.