In the New Grammar Book, I see that
An miste dom dul amach? (may I go out?) - has the answer Ní miste (you may) but I thought that ní was negative ?

miste itself is negative, being contracted from measa de.

An miste dom ... ? means "Is there any harm for me to ... ?" "Does it matter if I ... ?" so the answer must be in the negative Ní miste "there is no harm", "it doesn't matter".

It is a bit like the English "Do you mind if I ... ?" "No (I don't mind.)" = "Yes, you can".

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Feicim, go raibh maith agat.

ceist eile - in the phrase - Ní neart go cur le chéile - what's the word 'ní' doing here?

It’s the negative form of the copula. Go here means ‘until’ or ‘except provided that…’ or ‘without’. So ‘there is no strength until/withou/except providing you have putting-together’.

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Ceart go leor