I've just had this in the post - Clann Lir/The Children of Lir a bilingual publication from Mantra Lingua, which I have been trying to get hold of for quite a while. The story is retold by Dawn Casey and beautifully illustrated by Diana Mayo, but strangely they had to retranslate it back into Irish, would you believe - the translation was done by Máire Begley O'Shea. What surprises me is the use of 'Do' at the beginning of many of the sentences and in front of bí. Here are some examples -
They were blessed with four children - Do bhronn Dia ceathrar muirear orthu (and I didn't know the word muirear either).
Do bhí grá an domhain ag Lir dá chlann - The King loved his children more than anything in the world
Do bhí Fionnuala ag faire ar a leasmháthair - Fionnuala watched her stepmother.

muirear = family/children/offspring

As for the use of "do". Well, I don't think it's part of Standard Irish; I believe it's actually the reason for the lenition (adding of the "h") in the past tense. Anyway, it's still quite commonly used by Irish speakers. Do bhí = bhí.

I suppose the shorter answer is that it's just an older form.

That's a nice story. I remember it from when I was a kid. A lot of these legends were written in Old or Middle Irish, so that's probably why they had to translate it back into Irish, so modern readers can understand it.

Do bhí is either Munster dialect, or else an old literary form like Gumbi suggested. In some parts of Munster, “do” comes before all verbs in the past tense. In the Official Standard, it only appears when the verb begins with a vowel: d'ith (do + ith), d'oscail (do + oscail) etc.

Muirear is new to me. I looked it up, and apparently it means burden. I suppose that's a fairly accurate way to describe children.

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And here's another sentence that would be typically Munstrish, I suppose -
Is annso (?) a bhuaileadar le Evric, a chualaigh a scéal agus a chraobh scaoil é chun go mbeimis i ndán é innsint inniu.
which is translated as - It was here that they met the young farmer Evric (why couldn't they put his name into Irish?), who heard their story; and who told it. And so their tale was kept alive, and we tell it still today.

Annso, bhuailedar and beimis are all typical of Munster Irish.

Anseo has a broad s in Munster, that's why you get the spelling annso (or sometimes anso).

Evric should be Eibhric in Irish. I don't understand why they would use an anglicised spelling for the Irish version.

Yes. I believe beimis is spelled beimís too.

You're right, I missed the fada there - mbeimís - it is

Ar ndóigh, tá ceart agat, Gumbi. Is é beimis (gan fada) sa Chaigdeán, ach beimís sa Mhumhan.

Well if anyone's interested, you can still get a copy of it, if you look on the net, as on the Mantra Lingua website it's out of stock, though they do still have a few other books in Irish.
Clann Lir - The Children of Lir (Irish/English) by Dawn Casey, Diana Mayo and Máire Begley O'Shea
ISBN 978 1852698188 Mantra Lingua 2003 - it's available in other languages too - I have the Italian/English version (I Figli di Lir).