Many Celtic Place Names Don't Follow Gaelic Grammar

The Old European culture blog explores a curious aspects of many Irish and European toponyms in places in Europe that were historically Celtic at one point, as a side observation about Crom Cruah, a mountain which Irish pilgrims climb on the last Sunday of July (Reek Sunday):

Before you say that if Crom Cruach really meant Crom's Stack, Crom's Mountain, it would have had to be written Cruach Crom. This is because the Irish grammar says that when making compound words, you should always put adjectives after nouns. However there are lots of place names in Ireland that do not confirm to this rule. Place names such as Dubh Linn ("black pool" = Dublin) and Leixlip ("salmon leap") for instance. These place names were attributed to the Norse settlers who learned Irish had trouble with putting adjectives after nouns, so they often put them before the noun. This is exactly what happens when you force the new language on subjected population. They pick up the words but keep their own grammar. But this "incorrect" use of Irish grammar is present in all old Irish texts, which shows that it predates the Norse arrival to Ireland. For instance Táin Bó Cúailnge, is filled with epithets like finnbennach "white-horned", dóeltenga "beetle-tongued", echbél "horse-lipped", rúadruca "red-blushing", and the like.  

Funnily enough most toponymes and hydronymes of Celtic origin in central Europe follow this "incorrect grammar" and have adjective before the noun.

Here is an example: 

Gaelic word for “big” is Mór. (Pronounced as the English word more) 

Gaelic word for “river” is Abhainn . (Pronounced “awon” similar to the English word award). Proto celtic word is awa.

In central Europe there are numerous rivers called Morava. 

Morava = mor + ava = Mór Abhainn = Mor Awa= big river

Morava is the biggest river in Serbia and also in Czech republic, territories which were considered Celtic heartland. These rivers gave the name to the territory upper and lower Moravia.

In Ireland there is a river named the Avonmore River (Irish: Abhainn Mhor, meaning "big river") which is the same as Mor Ava just using Gaelic grammar.

There are at least two ways that this could be explained. 

One is that proto-Celtic didn't follow Gaelic grammar, because Gaelic acquired its grammar as a substrate influence on proto-Celtic from whichever (probably non-Indo-European) language was spoken before Gaelic in Ireland (possibly the language of the first farmers, possibly a Vasconic language spoken by the Bell Beaker people).

An alternative possibility is that Gaelic grammar, on a frontier where there was near total replacement and there was modest subsequent influence from neighboring languages, is the grammar of proto-Celtic and that the non-Gaelic grammar is the substrate that was replaced by Celtic languages.