Patrick wrote:

> A cethir = four (i.e. of the twelve sons)


> lia n-athair = le (with, by, at the hands of) + a n- (their) athair 
> (father)

Yes, but here I think "with" = "along with, together with".

> cona fodlaib = co n- + dat (with) a n- (their) fodlaib (dat pl of 
> fodail -
> distributions/portions)

It's clearly, as you say, dative plural, but "fodail" could also mean
"division, grade, rank (as of poets, kings, etc.), or it could even be
the dat. pl. of "fogal" (= raid, plundering, pillaging).  At least one
DIL example, "fodhla fedha", seems to use the word to mean "minor
(non-noble) classes" in regard to a hierarchy of tree varieties.

> I'm guessing this is in the sense of a father distributing land or 
> property
> to his heirs?

But does the context support that?  Having the whole passage makes 
interpretation easier, so here it is.  (After all, "location, location, 
location" is the most important rule in real estate; for us here, it
should be "context, context, context".)

> ¶430] Epscop Ibar trá ro baiste dá mac déc Bairr m. Cáirthind. A cethir
> lia n-athair cona fodlaib .i. Fergus, Nath Í, Énna, Áed. A cóic díb
> i m-Maig Réta: Fechuir, Bresal, Énna Dubbrónach, Fundech, Lugthigern.
> A trí díb h-i Feib .i. Éogan, Lugaid, Diarmait. Ad-róbart Barr a h-uu
> & a h-indhuu eter bíu & marbu do epscup Ibar co bráth.

... Four with their father and his/their dependants/retainers (?) ...;
five of them in M.R....;  three of them in F...;  Barr commanded his
grandsons and (great-)great-grandsons, both living and dead, to
Bishop Ibar forever.