Deasún

The form Deasún is a simplification of Deasmhumhain which is pronounced Deasúin NOT Deasún. But even if the spelling Deasúin were in use, it would hide the meaning which is clear in Deasmhumhain that it is deas "south" + Mumhain "Munster". It may be that the longer spelling may suggest a pronunciation dyasvuvin' rather than dyasu:n' but clarity of meaning is sufficiently important to preserve the older spelling.