I don't understand the use of "i" in this sentence: "Bhí fhios ag Colm go raibh an cur leis i mBeartla, freisin, ach é a bhaint amach."

Comhthéacs?

I'd guess (without context)
Colm knew that Beartla also had it in him to add to (?), but that it would have to be drawn out of him.

Anseo atá an chuid eile den abairt: "Agus ba fear i dteannta ag Beartla é, arae chuir sé i dtuiscint dó mura gcuireadh sé a phaisinéireacht chuige go mba thrua dó an baile a thaobhachtáil choíchin, agus bhí fhios ag Colm go raibh an cur leis i mBeartla......."

Beartla (Colm's brother) wants Colm to pay his way to America.

Vague memory of the story stirs....

Wasn't it the case that Colm was sending money home, and reckoned that he would get support by bringing Beartla over?

"i mBeartla" means that Beartla has some quality - in this case the quality of "cur leis" i.e. support.

In Hiberno English we would say that X has Y in him, or has it in him to do Y. I don't know if that is common english.

Actually, I think Colm is trying to discourage Beartla from coming to America, but I'm getting the sense that Colm feels he has to pay his way over, if Beartla insists. Do you think "ach é a bhaint amach." means "as soon as he (Colm) attained it (the passage)." in this context? Perhaps "an cur" refers to "the sending" (of the passage)?

Read a bit more, and see whether you agree with me! My recollection of the story was that while he was initially against Beartla coming over, he decides that it will make his life easier if he does.

I'm confident "cur leis" is a unit.

Maith go leor. Beidh mé ar ais go gairid.

Ceapaim go bhfuil an ceart agat, a Aonghuis. Go raibh maith agat arís!