[From Manx Soc vol 2, Kelly's Manx Grammar]
The Plurals of Substantives are formed of their singulars in three ways.
First, by adding only a syllable to the termination of the singular: as, awin, a river, plural awinyn; cassan, a footpath, plural cassanyn.
Secondly, by changing only the vowels or diphthongs of monosyllables into other vowels or diphthongs: as, mac, a son, pl. mec; fer, a man, pl. fir; beeal, a mouth, pl. beill; mair, a finger, pl. meir; or by changing the vowels and diphthongs of the ultima and penultima of polysyllables into other vowels or diphthongs; as, keeill, a church, pl. kiaulteenyn.
Thirdly, by changing the vowels or diphthongs of the singular and adding to the termination too; as, raantagh, a bondsman, pl. raanteenyn; claddagh, a lake, pl. claddeeyn; bleïn, pl. bleeantyn.
But here it is necessary to know the various syllables usually added to, or diphthongs changed in, the singulars of substantives, to render them plurals; which are these that follow:--
Yn is the most common termination of all; as, glioon, a knee, pl. glioonyn; laue, a hand, pl. laueyn; cass, a foot, pl. cassyn.*
The singular termination agh is always changed into ee; as, berchagh, a rich man, pl. berchee; kimmagh, a criminal, pl. kimmee; claasagh, a harp, pl. claasee.
Nouns, whose singular number ends in ey, make their plural by changing ey into agh, and adding the particle yn to the termination ; as, chengey, a tongue, pl. chengaghyn; caggey, a war, pl. caggaghyn; except dooinney, a man, pl. deiney.**
Some monosyllables ending in r make their plurals by taking aghyn; as, pooar, power, pl. pooaraghyn; gloyr, glory, pl. gloyraghyn.
A in monosyllables is changed for the most part into e; as, mac, a son, pl. mec; mair, a finger, pl. meir; so also tarroo, a bull, pl. terroo; marroo, the dead, pl. merroo; not terriu, merriu, as some erroneously hold.
E is changed into i; as, fer, a man, pl. fir.
O in monosyllables is changed into the diphthongs ui; as, molt, a mutton, pl. muilt; bolg, a belly, pl. builg; bock, a horse, pl. buick; poyll, a puddle, pl. puill; stoyl, a stool, pl. stuill; cront, a knot, pl. cruint.
* The old English or Saxon plural ended in en as house, housen; hose, hosen; eye, eyen; shoe, shoon. Hence also, sowen, now swine; cowen, now kine; oxen; men; women; children.--Ed.
** Other exceptions are--balley, a town, pl. baljyn; billey, a tree, pl. biljyn; bunney, a sheaf, pl. bunneeyn; carrey, a friend, pl. caarjyn; paitchey, a child, pl. paitchyn.--Ed.