Originally, Lonnath-Ernin might have been intended to mean 'royal havens', assuming the second element to be a regular adjective. However, the second element in Emyn Arnen 'hills of Arnen' is singular, and Tolkien later decided that it should mean 'Hill beside the water', see VT/42:17 and HL/119-124. Nevertheless, this meaning cannot apply to Lonnath-Ernin (havens are near water by definition), so unless we entirely reject this earlier form, we may assume that an adjective 'royal' is still possible.
Sindarin
arnen
place name. Beside the Water
arnen
adjective. (?) royal
arn
adjective. royal
emyn arnen
place name. Hills of Arnen
Some hills in South Ithilien (LotR/750). This name is translated “Hills of Arnen”, a combination of the plural of amon “hill” and the regional name Arnen (VT42/17).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name first appeared as N. Haramon (WR/359), later revised to N. Emyn Arnen (WR/363 note #3).
Emyn Arnen
noun. hills beside the water [see [His.], entry arnen]
emyn (pl. of amon “hill”), ar (#from Dor. ar - “outside, beside”) + nen (“water”)
arn
royal
arn (noble), pl. ern
arn
royal
(noble), pl. ern
ar
royal
(adj. prefix) ar- (noble, high). In the form ar(a)- this is an element in the names of the kings of Arnor and Arthedain.
ar
royal
(noble, high). In the form ar(a)- this is an element in the names of the kings of Arnor and Arthedain.
A name attested as an element in Emyn Arnen (LotR/750). In an essay on The Rivers and Beacon-hills of Gondor from 1967-69 (VT42/5-31), Tolkien indicated the name was incorrectly assembled by the local people from a mixture of Quenya and Sindarin elements: Q. ar- “beside” and S. nen “water”, thus meaning “Beside the Water” (VT42/17).