Originally Posted by EnhancedByCO

Unfortunately, the problem here is that the DUB-PHL flight now runs six days a week, except Wednesday. So there is no way for him to travel from Dublin on the 19th of December (a Wednesday), as US has no service from DUB on that day (own metal or codeshare).

That being said, if your attendance at the wedding is literally dependent on a a two hour time difference (the additional driving time from TPA instead of RSW), I would strongly suggest that you either move mountains to depart on the 18th, or take the refund and book a different carrier on the 19th. Leaving on the 20th, there is a reasonable possibility of winter storms in Philly delaying your travels such that you would miss the wedding entirely (at least, coming from DUB, you don't have to deal with an uncertain wait at immigration thanks to the pre-clearance offered).

Alternatively, you can see if they are willing switch your flight to an alternative European gateway (for example, Manchester or London). While you would need to make and pay for your own arrangements to get there from DUB (and risk a misconnect), you would at least be able to depart on the 19th.