Fòram na Gàidhlig

I figure (being German) he probably knows that...
I'm guessing he's taken /j/ as DH and delinited based on some model placename.

It definitely happens with common nouns (Norse halla (as in Valhalla)->Gaelic thalla->talla) so I it will have happened with many placenames, although curiously not always in the case of initial /h/ sound (Hiort, Hearadh and a handfull of names for island villages still feature an initial pronounced H).

Niall beat me to it. Gaelic routinely de-lenites loanwords that have an initial fricative into the nearest Gaelic unlenited equivalant. That is, if it doesnt just drop it.

For example:
hób > tòb (Western Isles)
holl > tallur
yawl > geòla
wall > balla
whip > cuip
wheel > cuidheall
votum > bòid
thraell > tràill
hogshead > tocasaid
George > Deòras

etc etc etc.

So the /j/ in Jórvik (my other half is from there, been there, done the ride would have been assumed to be the result of slender dh or gh. Hence Deòruig (or possibly Geòruig - but /j/ > g seems to have been less popular).

Place names are to *some* degree immune to that because they most commonly appear after a prep or in the genitive, where lenition is expected in Gaelic. But not totally. Deòruig would be 100% Gaelicisation. Dheòruig as a nominative would have been 75%ish. You get the idea.

Aidh aidh. I s'pose a lot would depend upon when the word was first known in the Gaelic. If from "Jorvik", then yes, the proses you said and Niall said could well have happened. But not from, say, "Eboracum", or even the later Welsh form "Efrog". So when and how.

Yes, Eboracum as a source would have ended up as the known Eabhrac or possibly *Eabhrach.

Not sure what Efrog would have yielded... LOL how did we get here?

Chan eil càil fhios a'm. Chan eil fiù 's fhios a'm cà bheil mi fhìn a-nis o chionn 's nach do thuig mi ainm an àite

Undermonkby? Àite faisg air Maoinchean... Unterpfaffenhofen

Neo 'm feum mi ràdh

A bheil na h-ainmean seo ceart?

Pirnmill (Arainn) - Muileann nam Piùirneachan
Catacol (Arainn) - Catagal
Brodick (Arainn) - Tràigh a' Chaisteil
Kilmarnock - Cill Mhearnaig

Glenfinnan - an e Gleann Fhionghain no Gleann Fhionnainn a th' air?

Tha am faclair aig Iain Mac an Tàilleir ag ràdh:

Brodick (Arran): Tràigh a' Chaisteil.
The English name is "broad bay", from Norse. This has been gaelicised as Breadhaig but is used only to refer to the part of the village known as Douglas Row in English. The Gaelic name means "the beach by the castle".

Catacol (Arran): Catagal.
This Norse name refers to a ravine, but the first part of the name is unclear. People from here were known as cuileagan-ime, "butter flies". The coastal area between Catacol and Lochranza is known as an Luirgeann, "the shank".

Glenfinnan (Inverness): Gleann Fhionghain. "Fingon's glen". There is a certain amount of confusion regarding the personal name here, because the area is also associated with Finan who is commemorated in Eilean Fhìonain, "Island Finnan", in Loch Sheil.

Kilmarnock (Ayr): Cill Mhearnaig.
"Ernoc's or M'Ernoc's church".

agus Pirnmill (Arran): Am Muileann or A' Mhuilinn. "Mill of the pirns". The Gaelic name is "the mill", with full name being Am Muileann Iteachan, "the pirn mill". Another name, Muileann nam Piùirneachan, "mill of the pirns", suggests that the English name came first and was later translated into Gaelic.

Cill Mhearnaig is right or wrong depending on which spelling convention you follow. All vowels before rn and rl are automatically long in Gaelic, which is why the older spelling didn't use the accent there, hence ard aird bard carn corn etc. If you follow the older convention, Cill Mhearnaig is fine.

If you follow the school that writes àrd àird bàrd càrn còrn, then it needs to be Cill Mheàrnaig.

Tapadh leibh!