This is another thing I'm having problems with - I understand what it means but I don't understand how it works and when you use it.
McGonagle says - it coalesces and in the plural it becomes lenár.
This is taken from a booklet about J K Rowling which is published by Storlann Nàiseanta na Gàidhlig
in the series Beatha le buaidh/Beatha le bua -
Nuair a bhí sí óg, ba ghnách léi a bheith ag sugradh lena deirfiúr Dianne.
Nuair a bhí sí ansin, bhuail sí lena fear céile Jorge Arantas, agus bhí iníon bheag acu, Jessica.

Is it the preposition "le" you're having problems with, or is it specifically "lena?" If you're asking about initial mutations, lena works just like a normal possessive pronoun.

Lena (with his) = séimhiú
Lena (with her) = nothing
Lena (with their) = urú

Lenár (with our) is the plural of lem (le mo).

There's some very details notes on the use of le here http://www.nualeargais.ie/gnag/le.htm

_________________
Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

It's when you use it instead of leis, léi srl - when is it leis an and when is it lena?

I made a couple of mistakes in my last post, that may have added to your confusion, but I've fixed that now.

An bhfuil tú ag teacht leis = Are you coming with him?
An bhfuil tú ag teacht lena chairde = Are you coming with his friends? (possessive form)

An raibh tú ag caint leí = Were you talking with her?
An raibh tú ag caint lena páistí = Were you talking with her children? (possessive form)

Does that help you?

_________________
Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

le becomes joined to the possesive adjective. le + a = lena, le + ár = lenár

le does not become joined to the definite article. le + an = leis an, le + na = leis na

Just to add a bit more to what Scooby said:

leis = le+é
leí = le+í
leo = le+iad

É, í and iad are the personal pronouns. A is the possessive pronoun (to say someone owns something).

_________________
Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

It is spelt "léi" and not "leí".

(Ulster and maybe other places: léithe)

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Go raibh maith agat, a Lughaidh

_________________
Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

In Munster both léi and léithi are common

_________________

Please wait for corrections/ more input from other forum members before acting on advice

Go raibh maith agaibh - so léithi would be the plural?