dia daoibh,

does this sentence make any sense?

"...de bharr sin, táim ábalta an feabhas a thagann orthu a fhéiceáil."

I'm trying to say, "...because of that, I'm able to see their improvement"

GRMA

"...de bharr sin, táim ábalta an feabhas a thagann orthu a fhéiceáil."

dá bharr sin, táim ábalta an feabhas a tháinig orthu a fheiceáil"

a thagann = present tense
and it's "dá bharr sin"

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I think an argument could be made for a thagann depending on the exact context. But in most cases it would be a tháinig as you said. (For example maybe they are ever-improving and it's only something you've been able to notice at a certain point and not before that).

Iontach go raibh maith agaibh. I was thinking along the lines of a contious type improvement, something that you see happening week in week out if that makes any more sense.