iRule on LC for SNAT

I have LC on which I want the out going traffic from a particular Private IP to get snated to a public IP. It should not be translated to LC self IP's. Is there a way for the following.

The SNAT pool (containing Public IP) which I will create for this Private IP to get translated to will contain public IP from both the ISP's but I want the out going traffic to go via ISP1 and shift to ISP2 only in case if ISP1 is down.

How can i achieve this, can any one help out in this please.

Regards,