This is the beginning of the story 'Rí na mBréag'

Do bhí baintreabhach ann, agus do bhí mórsheisior mac aici.
There was (once) a widow, and she had seven sons. (so far so good)

Bhí an cead mac comh ísiol san ná feudfadh capall Lá Márta breith ar mhullach a chod' gruaige, bheith ag inbhíor 'san pháirc is loime d'á bhfeacaidh súil riamh;

This is of course, a description of her first son. Why isn't it - an chéad mhac (with a fada on the 'e') ? I'm guessing that ísiol means low or short. Mullach a chod' gruaige is the top of his hair, capall, a horse, but what has it got to do with a day in March? páirc obviously is a field, but apart from that, I'm having difficulty in understanding the sentence as a whole, virtually none of these words are in the dictionary. Could somebody help me out please?

This seems to be Munster Irish.It is written in the language pre- Standard Irish "Caighdeán".If you have ever read Peadar Ó Laoghaire .ie. Séadna , Niamh etc you will see that this text is very similar.In "Caighdeán Gaeilge " ná feudfadh = nach b'fhéadfadh and d'á bhfeacaidh = dá bhfaca .
If you get a copy of "An Caighdeán Oifigiúil" there is a chapter in it how the pre-standardised spelling is modified into the caighdeán. It is worth a look.
Please remember that the best writing of the language and in the language was done before the 20th century and to appreciate and get a full picture of the language one has to grasp and understand the golden age of the language especially the "classical era" of the language .The era before the pre-modern .ie. before the 17th century.
One could express opinions ,idioms,etc in very short sentences in the language in the past that today requires much more words.

Thanks for that. Yes of course you're right there, it's one of the stories that are in the book 'Sgeuluidheacht Chúige Mumhan' compiled by Pádruig Ó Laoghaire. I imagine that the spelling must reflect the way they pronounce it. Sometimes they write his christian name - Pádraig - and sometimes it's Pádruig, so I don't know if there's a difference in the way that's pronounced. Apparently he wasn't related to Father Peter O'Leary. Unfortunately he died of TB when he was only 26 and on the title page it says Céad Cuid, so he didn't live long enough to compile any further collections, though he did have a great store of stories that he had remembered hearing when he was younger.
If you want to look at it, it's on Internet Archive and you can get a facsimile copy from Abebooks co uk and there's a transcription (with lots of errors in it) available on the RIA.ie website. Mind you there are lots of mistakes in the original version as well which makes me wonder whether the typesetter knew what he was doing.

This is complex, fair play for doing this well.

Ísiol does mean short, it's not a Munster spelling though, but an attempt at "Classical" Irish. This is a Munster text originally, which has been converted to the sort of "Caighdeán" they had pre-1950s.

Cead mac is a typographical error, in the original Munster text (1908) it's "chéad mhac", it's also missing an "agus" between gruaige and bheith, without which the sentence does not make sense. The edition you have is unreadable.

Overall:
The first son was so short that a young horse/foal wouldn't be able to catch the top of his hair, and him grazing in the barest field an eye ever saw.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Go raibh maith agat. That explains a lot. I've had a look at that edition - it's also on Internet Archive - and it does seem to make more sense. Does - Lá Márta - mean that the horse is young? or that it's a horse born in the Spring ?

Horses tended to be born in spring in Ireland (not sure about elsewhere), so a horse on a March Day would have been very young. It's sort of like "a spring chicken" in English. Also March is sometimes used as a shorthand for spring in Irish.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Ceart go leor - go raibh maith agat arís.