Ahem.
Modern Welsh rejoices in between eight and twenty different methods of plural formation, dependant on the speaker, dialect and analysis. Frankly, this is ridiculous and is more a reason that my native language needs to have a little word with itself than any whingeing about mutation or "not having words for yes and no". A synchronic analysis of these various pluralisation strategies invariably ends in folly, so I shall follow my instincts and preferences in offering a diachronic perspective.
Basically, alice and linguoboy have the right of it: they're essentially remnants of the varying stem formations in Proto-Celtic.
Sensu latissimo, we reconstruct nine(ish) stem classes in the language ancestral to Brythonic: analogous to the declensions of Latin or Greek. All of these are relatively synchronically straightforward, and I offer an example of a noun in the nominative singular and plural of each in the table below:
stem class | nominative singular | nominative plural | gloss |
ā-stems | merkā | merkās | 'maiden' |
o-stems | mapos | mapī | 'son' |
i-stems | vlatis | vlatejes | 'dominion' |
u-stems | katus | katowes | 'battle' |
n-stems | altrawū | altrawones | 'foster-father' |
t-stems | lukots | lukotes | 'mouse' |
nt-stems | karants | karantes | 'friend' |
s-stems | tegos | tegesā | 'house' |
r-stems | brātīr | brātres | 'brother' |
(There are also ī-stems, but I'm doing this from memory and I can't think of any. The existence of ū-stems is not relevant for Brythonic, and is probably a figment of Holger Pedersen's imagination.)
I've chosen these nouns for a reason. Look at their Middle Welsh reflexes:
stem class | nominative singular | nominative plural | gloss |
ā-stems | merch | merched | 'maiden' |
o-stems | mab | meib | 'son' |
i-stems | gwlad | gwledydd | 'dominion' |
u-stems | cad | cadau | 'battle' |
n-stems | athraw | athrawon | 'foster-father' |
t-stems | llyg | llygod | 'mouse' |
nt-stems | câr | cerynt | 'friend' |
s-stems | tŷ | tei | 'house' |
r-stems | brawd | brodyr | 'brother' |
So, we can see what happens: with the o-stems we see i-mutation from *-
ī. With the other classes, we can see that the final syllable is dropped in both singular and plural - the Middle Welsh plural forms are basically the oblique stems of Proto-Celtic.
However, the first of these is not like the others: the expected reflex of *
merkās would be
merch, the same as the singular. Instead, a form of the t-stem ending has been imported. Modern Welsh essentially ignores the diachronic origin of a noun and runs rampant with analogy. For example, the plural of
cath 'cat' is
cathod, but given the fact that it was originally a u-stem we would expect
cathau (which is indeed attested in Middle Welsh). However, it has taken the ending -
od (probably) from
milod 'animals'. In general, the most common plural suffix in Welsh is -
au (same in Corish with -
ow and Breton -
où), but u-stems were some of the least common nouns in Proto-Celtic.
Also,
llyg and
llygod are interesting here: in Modern Welsh the singular is not
llyg but rather
llygoden: it's a singulative. These are a Brythonic thing not found in Goidelic (afaik): they basically derive from Proto-Celtic diminutives.