Cirya has been the Quenya word for “ship” for much of Tolkien’s life, and is very well-attested with this meaning. ᴱQ. kirya “ship” first appeared in the Early Qenya Grammar of the 1920s (PE14/46, 79) and in the English-Qenya Dictionary of this same period, where Tolkien described it as the “general term” for a ship, as opposed to ᴱQ. lunte “boat” (PE15/77).
Tolkien regularly used ᴹQ. kirya “ship” in lists of noun declensions from the late 1920s and early 1930s (PE16/112-115; PE21/4, 46, 53). In one of these it was glossed “boat” rather than “ship” (PE21/53). ᴹQ. kirya “ship” appeared in The Etymologies from around 1937 under the root ᴹ√KIR (Ety/KIR), and it was derived from the root √KIR in later notes as well (PE22/150). It continued to be used in declension examples all the way up to the famous Plotz of 1966-7 (VT6/14).
The second version of Tengwesta Qenderinwa (TQ2) from around 1950 specified that its primitive antecedent had two variants: noun ✶kíryā “small swift sailing ship” and adjective kiryā́ “swift (especially of things that pass easily through obstacles)”, distinguished only by ancient patterns of stress (PE18/106). Hints of this second adjectival meaning can be seen in Late Notes on Verb Structure (LVS) from 1969, where Tolkien glossed kirya as an adjective meaning “swift gliding” in a note where he tried to distinguish the meaning of the roots √KIR and √KER (PE22/150).
Neo-Quenya: For purposes of Neo-Quenya, I would use cirya only as a noun for “ship”, and would assume its adjectival use “swift” was archaic. I would further assume cirya is used of faster ships with deeper hulls that cut through the water (especially if driven by sails or other motor), as opposed to a slower or flatter Q. luntë “boat” which moves mainly by floating on top of the water. Thus [in my opinion] the distinction between cirya and luntë is mainly the relative depth of the hull rather than the size of the vessel, so that luntë could refer to a large barge and cirya to a small but swift sailboat.
A noun for a “cleft”, most notably in Calacirya “Light-cleft” as mentioned in the Namárië poem (RGEO/62; LotR/377).
Conceptual Development: In Silmarillion drafts from the 1930s, this name was ᴹQ. Kalakilya (LR/173; MR/102), and the word ᴹQ. kilya “cleft, pass between hills, gorge” appeared in The Etymologies of the 1930s as a derivative of the root ᴹ√KIL “divide” (Ety/KIL). The word kilya also appeared with the gloss “chasm” in Lament of Atalante from the 1930s and 40s (LR/47, 56; SD/247, 310), but at some point when composing the final versions of the Namárië poem Tolkien switched to Calacirya, and he made the same change in later versions of Silmarillion drafts (MR/102).
Neo-Quenya: I’d avoid this word for Neo-Quenya, as it is too easily confused with cirya “ship”.