Wife's dependent visa expires 30 Dec 2018 but we leave the UK mid-Feb. Worth renewing?

I have a Tier 2 General visa expiring in June 2019 but my wife's PBS Dependant Leave to Enter Residence Permit expires 30 Dec 2018. We are both American and are relocating to Ireland sometime in February for my work. To extend her visa I believe we need to pay the £600 IHS charge + application fees. Since she can visit the UK on an American passport for 6 months is it even worth renewing her visa for the extra 1.5 months, assuming she does not work?

Coincidentally, we return to the UK following a holiday trip home on 30 December. Since her Residence Permit expires on 30 December can she simply enter the UK on her US passport or would she need to enter the UK at some point from 31 December? We can easily change our return date to 31 Dec.

in this case, your wife shouldn't renew the visa for sure.

After her visa expired, she can still come to the UK and just need to have valid return ticket and show that she will leave the UK, also travel insurance in place. She will be asked what's her plan in the UK at the border.

I think you will also be in the application of Ireland visa, both you and your wife. Even better if both of your visa in ireland get done.