This part of the sentence seems like it's missing a predicate. i.e. 'Here went some of the girls that were at the beach informing those who were on the field that all of the women who were on Oileán Bán ....' Is this just a typo? The only possible explanation I could think of is '... that all of the women that were were on Oileán Bán', but if that really were the intended meaning I would expect something instead like go raibh a lán ar na mná a bhí ann ... or go raibh (a lán) a raibh de (sna) m(h)náibh ..., an both of those are pretty clunky.

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

go raibh a lán ar na mná = lit. "that was much on the women"

There is "ar" (partitive would be "de": a lán de na mná = many of the women)

Could you perhaps give a breakdown of what this sentence actually means? I am still confused as to how it cannot have a further predicate.

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

I'd need to see more of the context around the sentence, but I think Labhrás has answered it for you.

Literally - "informing those who were on the field that there was a lot on the women who were on Oileán Bán"
I assume that 'ar na mná' is used in the same sense as in English to express obligation(s), so maybe the non-literal meaning is that the girls were saying that the Island women had a lot going on and were super busy.

The sentence is grammatically complete.
It has a predicate (raibh … ar na mná) and a subject (lán).
The meaning must be deduced from the context.

There is "lán" (not "a lán") in this version:
https://wikisource.org/wiki/Page:An_t-oileanach.djvu/42
So, it does not mean "a lot" but "(the) full".

And in the context of the rising tide, it is probably "lán mara" (full of sea, a synonym for high tide)

bí ar has many meanings, so unfavorable conditions can be "on" someone.
Here it is "lán", "full" (high tide) which is "on" the women.

The women had to endure the tide and had to stay on Oileán Bán until the next morning.
(B’éigean dóibh fanúint mar a raibh aca. Dubhairt a raibh timcheall gurbh’ é a stáitse go maidean é, a line before)

In the English translation Islandman , it is:

Thank you for clarifying, I guess I must have misread

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar