You should change your login mapping to something like:
RKObjectMapping *loginMapping = [RKObjectMapping mappingForClass:[Login class]];
[loginMapping addAttributeMappingsFromDictionary:@{ @"success":@"success", @"message":@"message", @"data.user_id":@"intUserID", @"data.firstname":@"strFirstName", @"data.lastname":@"strLastName"}];
RKResponseDescriptor *responseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:loginMapping
method:RKRequestMethodGET
pathPattern:@"PathTOLogin"
keyPath:@"result"
statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
Basically, remove 'data' from the key path of the response descriptor and instead use it in the mapping key paths.