The company (in Ireland) said I had agreed to the DCC, accordingly there was no "billing error" and I couldn't dispute. As I see it (a) I have no contractual relationship with the company in Ireland and did not agree to the DCC -- which was what all this was about -- and therefore their rule didn't apply, and (b) I do have a contractual relationship with my credit card issuer and it says I can dispute.

But -- when I began researching this, I found that MC and VISA require the cardholder to be informed in advance and to agree to the DCC, and if that's done they will not accept a dispute. The catch appears to be that in some cases the "consent" is hidden (or in mouseprint possibly in a language other than yours) on the thing you sign for the charge.

In my case, as I had stricken through the "conversion" before I signed the invoice reconciliation, there was no way they could seriously contend I had agreed. I suspect, though, that anyone who signed the reconciliation without striking through the bogus conversion could be argued to have consented. This sort of "ambush" is why I'm concerned enough to post this as a separate thread.

Edited to add: The main reason I didn't simply dispute with the credit card issuer (although I would have as a last resort) is that my dispute would be only for the delta between what the credit card issuer would have charged and the amount the Courtyard -- or the company in Ireland, whichever -- charged, so about 3% to 5% of the bill. Most credit card companies have a "nuisance limit" where (unless the cardholder has been "abusing" the dispute right) the credit card company finds it cheaper to simply grant the credit for the disputed amount rather than to take the administrative time and energy to actually do a charge-back. I was concerned that the credit card issuer (Chase) might just grant me the few dollars in dispute without charging back to the Courtyard, so they would keep their additional "take" from DCC. In my mind, what they did was wrong, and they needed to disgorge the additional fee they took without just cause, if for no other reason than to discourage them from doing this to other people without getting their informed consent.