I sing with a choir. Recently we started working on an English version of the song "The Garten Mother's Lullabye" which contains one allegedly Irish word: "Leanbhan" for "child". I thought the word was "leanamh"?
Could this be some (perhaps Northern) slang version of the word? Or is it an outright mistake?
Also, is there a version of the song in Irish? Where could I find the text for it?

No, it is a diminutive/pet form

Leanbh & án

I found some Irish Lyrics here:
http://www.mudcat.org/thread.cfm?threadid=11380


Codail, a linbh, le crónán na mbeach,
Tráthnóna ciúin atá ann.
'Sí Aoibheall a thagann ón liath-chreag ard
An tír fé chlúid dá báthadh.
A linbhín óig, mo chuid den tsaol,
Is tusa rún mo chléibh,
Ó chanann an criogán an suantraí sámh
La hais na tine tréith.

Codail, a linbh, le suaimhneas id' chroí,
'S ná bíodh ort brón ná crá.
Tá Dia id' fhochair sa chliabhán fós
Ag faire ort ann go lá.
Tá mise led' thaobh ag guí duit fós
' Tabhairt síochána chuig mo rún,
Is aingil na bhflaitheas ag eitilt thart
Ag cur mo ghrá chun suain.

However, these may be translations from the English.

Leanbhán, again

http://www.smo.uhi.ac.uk/gaeilge/focloiri/daoine/lorg.php?tairg=Lorg&facal=leanb h%C3%A1n&seorsa=ceannfhocal

Go raimh maith agat.

Does the vocative change it to "a linbhín"?

I would rather think that it is leanbháin.

I'd agree

"Linbhín" in the text above is a different, related form.

So, pronunciation would be roughly; "lyAnvawn"? The text we have has no fada, but I'm thinking that is a mistake.

quote:

The text we have has no fada, but I'm thinking that is a mistake.

Yes.

You can listen to it at abair.ie, for example.

The Phoneme engine there renders it as:

sil # lʲ a n̻ˠ ə vˠ a n̻ˠ # sil #

(Greek to me, but some linguist may be able to make sense of that for you)