Jiangsu, HS1972 wrote that he is from Sweden. That means his DC (issued there) has little to with your DC (assuming it's issued in North America).

Diners Club is a franchise. And it's implemented quite differently in every country.

US / Canada DC, which runs on the MasterCard netowrk, is the only one being sold to Bank of Montreal.

OTOH, the Diners Club network (for those cards which are not on the MasterCard network, like the ones in Sweden). has been bought by Discover, which is merging the Diners Club transaction processing network into the Discover Card transaction processing network. It's that latter change, which again has nothing to do with BOM (as far as we know), that is likely causing all the havoc in Ireland.