Domhanda is an adjective that can be interpreted as "global" (téamh domhanda - "global warming") or "worldwide" (an Gréasán Domhanda -"the World Wide Web"). In that sense of "involving the whole world", it is also used in an Dara Cogadh Domhanda - "World War II" and ceol domhanda - "world music".
On of the functions of the genitive is to allow you use a noun as an adjective - an envelope is a clúdach litreach and you can interpret that as "a letter cover", "a cover of a letter", "a letter's cover" (ie as an adjective, using "of" or using the possessive 's). Other examples are Parlaimint na hEorpa, where the genitive na hEorpa is usually interpreted as the adjective "European" rather than "of Europe" or "Europe's" (though both of those interpretations are also correct, and may be preferred in other contexts) or i lár na cathrach which can be "in the city centre", "in the centre of the city" or "in the city's centre" (though "city centre", "center city" and "central city" may be a bit too idiomatic to be a good example).
So both Corn an Domhain and an Corn Domhanda are used. It might depend on whether you prefer to think of the holders as "Champions of the World" or "Global Champions".