"Tnuthán an Dúchais" arís! "Ná níor lig sé an bhá lena ais, gur inis Nóra Mhaitais Labhráis dó oíche amháin, i gcois íseal, gur tháinig scéala i litir chuig Joe Thuathaláin go raibh fógraithe Cáit Anna agus a fear a chur as seilbh faoi bheith ina ndrochdhíolaithe cíosa le dhó nó trí de bhlianta anuas." Agus cén fáth a bhfuil "Ná" ag tús na habairte?!

This must be following another sentence dealing with the same subject.
Ná in this case = Nor:

Nor did he let sympathy near him, until Nóra M L told him one night, quietly, that news came in a letter to Joe T that it had been announced that Cáit A and her husband would be evicted for being bad at paying the rent for two to three years past.

Slightly too literal translation...

Go raibh míle maith agat arís, a Aonghuis. The literaler, the better. It helps me see the connections. "i gcois íseal" means "quietly"? And "gur", "until"?

I'll confess to not knowing the exact meaning of "i gcois íseal" - but I'm fairly sure that it means quietly/in a quiet aside.

gur doesn't always mean until - it does here.