Technically, it's a "suffixed conjunctive pronoun ... to support or emphasise a prefixed or infixed pronoun,"* e.g. fy nghariad innau - my (own) love.   In spoken form, the 'fy' is quite often dropped and 'innau' becomes 'inne' in some dialects.

The conjunctive form is particularly emphatic (hence my addition of 'own' above). If the preceding word ended in 'f' it would be 'finnau'; or if in a vowel, 'finnau' or 'minnau'.

a.k.a. Emphatic Pesonal Pronouns.

Thus endeth the lesson ...

sian

* A Guide to Correct Welsh, Morgan D Jones