First of all - was the irrigation leaking?
Second, how is your contract worded?
Third, how much is the bill?
State law varies regarding who is at fault/responsible. Ordinarily it is the homeowner's responsibility to monitor their utility consumption. This said some states require a warranty on system install which may include contractor responsibility - but it all depends.
Personally, if the homeowner waited five months and is now screaming the first question I would have is what didn't you call before?
Provided the bill isn't excessive, say under $300 bucks and depending upon your relationship with the client - I'd say maybe discuss this with them.
Nice Try = You Suck Spelled Differently