The trends of my naming conventions are rough but follow a sort of convention. This is meant to reflect changing attitudes and preferences in the management of the NWR, and how they want to present themselves.
I've added notes below in italics to explain the thinking behind each era.

Passenger/Mixed traffic locos

Manx/Sudrian authors, poets, notable historical figures - mainly locos that were there when the NWR was formed 1915-1920
e.g. Edward Faragher, Thomas Allen
*William Montagu is the exception to this, having arrived in 1931. Maybe it took the name of a scrapped loco?
The newly formed NWR, providing at long last a fixed link to mainland England, wanted to establish itself as devotedly Sudrian and to not represent 'mainland control' over the island, which has always been fiercely resisted. As such, locos present in 1915 were often named after local Manx and Sudrian heroes and notable people.

Sudrian legendary figures - 1920s-1930s
e.g. Thorkell, Sigrid, Godred Crovan, Thorfinn the Mighty
Similarly, the NWR wanted to celebrate Sodor and Man's history and legendary tales.

Contemporary Sudrians - 1930s-1940s
e.g.Sir Albert Regaby, Rebecca Qualtrough,
*Colonel Henry Regaby is an exception, the loco having been named in 1915, however the second loco continued the tradition in 1935
During the 1930s, the depression hit. There was a need to remind the populace that people such as themselves could make a difference, and not just characters from legend. The war brought a renewed sense of national pride and a will to celebrate the efforts and sacrifice of local people.

Sudrian Icons (buildings, landmarks etc,) - 1950s
The post-war period was marked by a will to build a new society, and the management very much wanted to be a part of this, while retaining a sense of Sudric Pride.

Goods locos (most railways did not name goods locos, but this one does, and it lines them out!)
Warships built at Barrow - large locos
e.g. Revenge, Illustrious, Triumph
Courting business from Vickers-Armstrong, the NWR was a railway born in the fires of war and owed its existence to it.

Submarines built at Barrow - small locos
e.g. Perseus, Proteus

Electric locos (1500v DC working the Peel Godred line)
Sudrian geographical features - 1920s-1930s
e.g. Corloey, Dubbhyn Moar
Peel Godred had long been neglected and naming the locos after local landmarks which in turn gave the railway the hydro-electricity it needed seemed an obvious choice!