The vast majority of placenames in Ireland are anglicisations of Irish language names; that is, adaptations of the Irish names to English phonology and spelling. However, some names come directly from the English language, and a handful come from Old Norse and Scots. The study of placenames in Ireland unveils features of the country's history and geography and the development of the Irish language. The name of Ireland itself comes from the Irish name Éire, added to the Germanic word land. In mythology, Éire was an Irish goddess of the land and of sovereignty (see Ériu).

In some cases, the official English or anglicised name is wholly different from the official Irish language name. An example is Dublin: its name is derived from the Irish dubh linn (meaning "black pool"), but its Irish name is Baile Átha Cliath (meaning "town of the hurdled ford").

For most of the Gaelic period, there were very few towns or large settlements in Ireland. Hence, most places were named after noteworthy features of the landscape, such as hills, rocks, valleys, lakes, islands, and harbours. As time went on, more places were named after man-made features, such as churches, castles, and bridges. Some of the most common elements found in Irish placenames are shown in the table below. The differences in spelling are often due to differences in pronunciation.

During the 800s and 900s, Vikings from Scandinavia raided monasteries along Ireland's coasts and waterways. The Vikings spoke the Old Norse language and are also called Norsemen. They set up small coastal camps called longphorts – these were used as bases for their raiding parties and as shelters during the winter. Eventually, some longphorts grew into Norse settlements and trading ports. The biggest of these were Dublin (which became a Norse-Gaelic kingdom), Wexford, Waterford, Cork and Limerick. Over time, the Norsemen embraced Gaelic language and culture, becoming known as the Norse-Gaels (Gall-Ghaeil in Modern Irish, Gall-Gaidhel in Old Irish).

Placenames derived from Old Norse:

After the Norman invasion of Ireland, which began in 1169, Anglo-Norman and English language placenames emerged in the areas under Anglo-Norman control. Most of these are within the bounds of "The Pale" – the area that stayed under direct English control for the longest, and where English language and culture held sway. It stretched along the east coast from Dundalk in the north to Dalkey in the south.

Between 1556 and 1641, during its "conquest of Ireland", the English colonised parts of the country with settlers from Great Britain. This is known as the "Plantations of Ireland". After the 1601 Battle of Kinsale defeat in which the Gaelic aristocracy fled to continental Europe the northern province of Ulster was the most heavily colonised. Those who settled as part of the "Plantation of Ulster" were required to be English speaking made up mostly of Lowland Scots and some northern English. The result is that northeast Ulster also has a great number of English-derived placenames.

Such placenames often refer to buildings and other manmade features. They often include forms such as -town, -ton, -ville, -borough, -bury, bridge, mill, castle, abbey, church, etc. However, forms such as hill, mount, mont, wood, bay, brook etc. are not uncommon.