Firex smoke alarms offer a relay module
http://www.firexsafety.com/NR/exeres/A0CBB706-8A6B-468A-A11C-EC600BA7D871.htm
As far as a siren you can get 120 volt models from any place that sells security systems.
Watch out for temperature limitations of smoke detectors. You would not want to be held accountable if this does not work and they lost live stock.
iWire,
Good product link. I like the idea.
In Shelby County, we have more horses than people in some areas. I'd never thought about this before.
I wonder, since all the Smoke Detectors are interlinked, how about mounting another one outside (under a little awning)?
But, maybe the SD is not listed for outside use.
I guess one issue is that the EC is guaranteeing
that it will work!
The OP could run a 14-3 across into the house,
and interconnect with the residential SD.
FWIW, I have measured the interlink voltage transferred, and it is the same as the battery voltage (9V). Which makes sense, because they are supposed to work off the batteries.