Except that, as I noted above, in Hiberno-English "They have the child frightened" is a perfectly reasonable construction, and is basically a direct translation from the Irish phrase.
(It would usually be used in a slightly chiding tone - "You have that child all excited about his birthday", "he has that dog exhausted chasing after that stick", "he has his poor wife driven mad", "he has the house painted from top to bottom", "You have that poor boy scared out of his wits with your ghost stories", etc)
From a purely grammatical point of view, you can say that Tá an leanbh scanraithe acu is "they have the frightened child". From the point of view of normal, idiomatic usage, though, that would be an unlikely translation, except in a very specific context, and the more obvious translation is "they have frightened the child" or "they have the child frightened".
In other words, while both the Irish and the English sentence use language that normally indicates possession (tá .. ag and "have"), possession of the child would not be the default interpretation of this sentence in either language. If you wanted to clarify that "they have the child with them" is the intended meaning, then you could say tá sé acu, an leanbh scanraithe or tá an leanbh scraithe leo.
I'm wondering - to "have frightened" means using the word scanraithe as a verb, to me.
Doesn't the sentence you give translate to more like, "the child is frightened by(with) them", I don't know if it's a valid sentence not being a native speaker. Or if it's the right prepositional pronoun to use. Thanks for any help!
The literal translation is "they have the child frightened" (which happens to make perfect sense in Hiberno-English). "They have frightened the child" is a better translation than "the child is frightened by them" because the child isn't the subject of the sentence.
The verb is Tá, but it's Tá ... acu, which is the equivalent of "to have" in English.
I'm not sure that linguistics and grammar are all that helpful in this case, because the construction of the sentence in English is quite different from the construction in Irish
Tá an leanbh scanraithe - "The child is frightened"
Tá an leanbh scanraithe acu - "They have the child frightened"
This is just one of those idioms that doesn't have a direct translation. Tá ... ag usually means possession/ownership of something, and in for this particular sentence, there is no way that I can think of to say this in English that implies possession (except in the supernatural sense, but sin scéal eile!). The "have" in "they have frightened the child" isn't the "have" of possession/ownership, but "They have frightened the child" still gives a better sense of the meaning of the original sentence.
But on another level, you could reasonably make the case that "The child is frightened by them" and "They have frightened the child" mean the same thing, and it's irrelevant which is the subject and which is the object.
See and I thought the possession would end up making it "They have the frightened child.", I am still floundering then if this ends up making the present perfect. Did they cause the child to be frightened or do they simply have the child who is frightened with them?
To me, there is a difference between the standard English They have frightened the child and the Hiberno-English They have the child frightened. The first focuses on the action, the second on the state of the child after the action. The first frightens the child, the second sees her in her frightened state.