As sin a tháinig
A literal translation of this to English would be "From there it started".
Is this an idiom and what is the equivalent in English? Can you put it in a good sentence for me please?

I don't think you'd call it an idiom. It is a normal phrase.

.... as sin a tháinig an nós coinnéal a lasadh Lá Muire.
.. it is from that the custom to light a candle on Mary's Day started.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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Yeah...this one seems like a pretty straightforward translation rather than an idiom. Idioms tend to be a bit more obscure when you try to translate them...something like "cuireann sé isteach orm," which makes perfect sense in Irish but kind of tangles up your brain if you try to make it work literally in English.

Redwolf