I posted the other day that PUL had deich céad míle for "a million", but note: no eclipsis on the céad.

Then in Niamh there is this:

“Tá chúig céad déag fear againn cheana, a athair,” arsa Tadhg Óg, “agus tá fhios agam go mbeidh chúig céad eile againn sar a’ fada.”


I a wondering if a general rule can be formulated as to when céad resists lenition and eclipsis? Mo Sgéal Féin has 1602 as "sa bhliain d'aois an Tiarna míle sé chéad a dó".

Diarmuid Ó Sé (section 449) says of Corca Dhuibhne that the words acra, céad, clais, lá, mí, míle, oíche, seomra, slí and other words take the singular form after 3-10. In section 450, he says of initial changes after 3-10, that the only definite rule that can be laid down is that a noun in the singular form beginning with a consonant takes lenition after 3-6. He says of "céad" that the initial c is often eclipsed after 7-10 but that it is more commonly lenited.

yes, céad is definitely in the singular. You can't have "sé céadta".

So it ought to be lenited in the cases I illustrated. Well, that is what you deal with in old books!