Hi Aelfarh,
I tried to source further for you but I can find no attestation to
aislingthe as "a journey to the sky and the stars" as a group name-class for rituals of dreaming. I don't know from where that is derived that (if there is a specific attestation of such) or whether that's an inferred meaning. It may well be there is that word for a class-description but my Old Irish isn't good enough and I can find no reference for it (that may say more about my search-possibilities than its existence or lack of!) so maybe if Ms Rowan Laurie happens across this thread she can add to it.
Some adjectives can have a -the / - thae suffix in Old Irish eg. seanathar
thae - 'grandfatherly' for sean-athar
dae(Milan glosses 99b8), corpthae/corpdae 'Bodily', but usually it's -de/-dae (eg. nemdae 'heavenly) but the word aislingthe would not be an adjective as in her book it's being used as a collective noun term for a group of rituals to do with dream. I don't know whether there is a direct source of that.
Collective nouns in old Irish are often formed from suffiexes like - red/rad, -er/-ar, -bad, -ten/-tan (or then/-than) (Ref. Grammar Old Irish, Rudolf Thurneysen sections 263-5.
The Dictionary of the Irish language (which is online at
www.dil.ie) gives
aislingthe as a later form of the
nominative singular io-stem "
aislinge" with intrusive -th-, ditto in the dative singular
aislingthi . There are journal references to both occurrences of those forms so if your uni has a celtic studies section have a look at these:
ZCP viii 217 section 10 (Zeitschrift fuer Celtische Philologie
Ériu ii (1904) 204.2
If aislinge is originally an -io stem then the nominative plural noun I think would be expected to end in -i if masculine eg. aislingi or if neutral in -e just as the nominative singular. (modelling on masculine n.s céile and neuter n.s cride respectively).
Aislingthe is attested several times in
Saltair na Rann, a long Middle Irish poem, but translation here is as nominative singular for dream (
aislinge with intrusive th I guess) rather than a class of magical journies
(Translation by Prof David Greene available on DIAS link:
http://www.celt.dias.ie/publications/on ... r_na_rann/ }
eg. Lines 3277/3278: Bae in rí fo glámaib gné / do dálaib a
aislingthe"The king was under appearance of censure/on account of his dream"
Lines 3349/3350: Cor fallaigder duit cen chess/ int
aislingthe laind lán-des
"So that the pleasant beautiful dream may be made clear to you without trouble!
So a little more light on its forms in various uses but I cannot find anything to attest it as a group noun for rituals of dreaming involving visits to sky/stars. I see it mentioned as such in "Weaving Word Wisdom" (a very nice book) but no reference information for that derivation is supplied.
best wishes
Beith