I replaced my ford sierra IRS 1992 model alarm batteries now the car wont start

With the key in the ignition. Disconnect the negative battery cable, wait 15 or more seconds and reconnect the battery cable. That should reset the alarm.

Very common story, your electrical system took a shook and has forgotten its memory, that means it thinks that it is stolen, no shortcuts sorry, it has to go to a dealer, if you ring them they may be able to come to you
Merry Christmas

Sounds like alarm module has failed. Call ford parts and ask if common failure, if so replace. You could disconnect the alarm module and see if the car starts.

you need to reset the immobiliser/alarm.

disconnect the battery for 1 minute and try again - ---quick fix possible with that---
if that makes no difference then I would like to know exactly what model of alarm and immobiliser the car has - what is the maker + model no/serialno of the keyfob for the alarm, I can get the info I need off that.
also you can check the fuse in the alarm if you know where the alarm is situated, it will have a fuse either on an inline fulseholder on as a spade fuse stuck into its casing somewhere - replace the fuse if blown and try again to start the car ------------ tell me the model no of your alarm and immobiliser if no joy after battery/fuses and I can read up on them and see if I can fix your problem...

Have you tried turning the alarm back on so that it goes off again, lock and unlock the car with your fob now, then enter the code. This is because it might still be in the standard immobiliser mode.

has the alarm flattened the battery? or does it have anti-theft immobiliser

try unhooking the battery while the battery is disconnected turn the ignition on leave the ignition on while you reconnect the battery this should override the alarm

There is a way to override the code, however you need the code number. The only other way is to sit in car with door shut, play with the central locking button on dash, locking and unlocking. While doing this occassionally turn the ignition on and off. If you do it long enough, you may find the immobiliser light goes out, if it does, quickly start the car.

TL

if not the key probably the reciever in the ignition switch for the immobilizer transponder