Omagh is a town in Northern Ireland. Most of the world learned the name in 1998, when a bomb planted by a group calling itself the Real IRA killed 29 people there — one of the worst single attacks of the Troubles. In English news the gh is not a hard g; you hear something like “OH-mah”. Outside Ireland, people almost only say the word when they are talking about that bombing, or about the town itself. It is a place name, not a word you pluralise.