Another claim made by Daniel Cassidy in his ridiculous work of pseudolinguistics, How The Irish Invented Slang, is the claim that the terms mammy and daddy (and ma and da) have their origins in the Irish language with mamaí and daidí. I don’t intend to spend too much time on this, because it is laughable and anyone with a basic knowledge of linguistics would realise that it is nonsense.
Terms like ma or mama for mother are found in many parts of the world and in many languages. Many linguists believe that because these are the first recognisable sounds that children tend to utter when babbling, they have become attached naturally to the meaning of mother. A useful overview of terms for mum and dad can be found on Wikipedia at: http://en.wikipedia.org/wiki/Mama_and_papa
Etymonline says this about the origins of mamma, the original form of mummy in English:
“1570s, representing the native form of the reduplication of *ma- that is nearly universal among the Indo-European languages (cf. Greek mamme “mother, grandmother,” Latin mamma, Persian mama, Russian and Lithuanian mama “mother,” German Muhme “mother’s sister,” French maman, Welsh mam “mother”). Probably a natural sound in baby-talk, perhaps imitative of sound made while sucking.
Its late appearance in English is curious, but Middle English had mome (mid-13c.) “an aunt; an old woman,” also an affectionate term of address for an older woman. In educated usage, the stress is always on the last syllable. In terms of recorded usage of related words in English, mama is from 1707, mum is from 1823, mummy in this sense from 1839, mommy 1844, momma 1852, and mom 1867.”
As for the English word dad, this is what Etymonline has to say:
“recorded from c.1500, but probably much older, from child’s speech, nearly universal and probably prehistoric (cf. Welsh tad, Irish daid, Czech, Latin, Greek tata, Lithuanian tete, Sanskrit tatah, all of the same meaning).”