Dia dhaoibh,

My understanding is that the dialects of Ulster and North Connaught retain the four Ls and Ns of Old Irish.

Ls

Velarised dental variety : /L/
Velarised alveolar variety : /l/
(and their palatalised counterparts)

Ns

Dental variety : /N/
Velarised alveolar variety : /n/
(and their palatalised counterparts)

I've seen them referred to as:

single l /l/
double l /L/
single n /n/
and double n /N/.

Looking through some some dialectal studies I've noticed a general correlation between the orthography and pronunciation i.e.
buile = /bil'ə/
buille = /biL'ə/

Are there other guidelines? What about initial Ls and Ns? How do you know if they are dental or alveolar?

Hopefully someone can shed some light on this!

Grmma

Initial unlenited Ls and Ns are tense /L/, /L′/, /N/ and /N′/
Initial lenited Ls and Ns are lax /l/, /l′/, /n/ and /n′/.

True.
Also, in clusters, normally n's and l's are dental too : comhairle is /ko:rL'ə/, tairne is /taːrN'ə/, etc.

_________________
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Agus is í Gaeilg Ġaoṫ Doḃair is binne