1. For a general sign requesting people not to do something, is it usually in the singular or plural? Is there an agreed standard?

Do not cross the foul line = 'Ná trasnaigh an teorainn' or 'Ná trasnaígí an teorainn'?

2. http://www.focloir.ie has 'líne fill' for foul line and http://www.tearma.ie has 'teorainn' for foul line. It is for a bowling alley. I think I prefer 'teorainn', but if someone tells me the other is more accurate, I'll pass it on.

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Ná trasnaítear ...

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While I'd lean toward Labhrás's wording, I remember having this discussion years ago on the old forum, and the consensus was you used the singular because each person who looks at a sign is being addressed as an individual.

Redwolf

Always impersonal "Ná trasnaítear"

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Definitely impersonal imperative for "official" looking signs and warnings.

Singular imperative for less formal notices, etc., such as on the net, etc. (but even then using the impersonal imperative will increase the "weight" of the order.)

Plural imperative when you are addressing a group collectively, such as to a family in a greeting card, etc.

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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).