This is a list of the longest place names in Ireland. It includes names written in English as a single word of at least 20 letters. The vast majority of English-language place names in Ireland are anglicisations of Irish language names. The spelling which has legal force is usually that used by the Ordnance Survey of Ireland. Many of Ireland's longest place names are found in the far west of the island, where the Irish language survived the longest; including Gaeltacht areas, where it is still the vernacular. These names are generally written as multiple words in Irish.
At 22 letters long, Muckanaghederdauhaulia (from Muiceanach idir Dhá Sháile meaning "pig-marsh between two saltwaters") is often believed to be Ireland's longest one-word place name. In by Georges Perec, it is one of the ports visited and painted by the character Bartlebooth, who believes it to be the longest port name in the world. An 1830s boundary survey uses the spelling Mwickanaghedderauhaulia, which is 23 letters long.
However, the Placenames Database of Ireland (logainm.ie) records several other place names of at least 22 letters. Furthermore, the anglicised form Muckanaghederdauhaulia ceased to have official standing in 2005, after a Placenames Order was made under the Official Languages Act 2003; the sole official name is now the Irish form Muiceanach idir Dhá Sháile.
Multiple-word place names may be longer still; for example, "Plughoge and Leabrannagh Mountain North", the name of a townland in County Donegal, has 35 letters, and its Irish equivalent "Sliabh Phlochóige agus Leadhb Reannach Thuaidh" has 41.