In English, the preposition by is used for announcing the agent of an action in passive constructions: it was done by him (passive) corresponds to he did it (active). In Irish, the story is more complicated.
The author of a book or an article: “A Tale of Two Cities by Charles Dickens”: Use le, if there is no verbal construction involved: Scéal fá Dhá Chathair le Charles Dickens. (Of course, it should be Scéal faoi Dhá Chathair in standard Irish, but the existing translation comes from the pre-Caighdeán era, and the translator was an Ulster dialect speaker.)
When the action is (or was, or will be) ongoing and not completed, we use the construction with do + possessive + verbal noun. This passive construction takes the ag agent.
Tá muid dár n-ionsaí ag trúpaí naimhdeacha “We are being attacked by enemy troops”
Bhí sibh do bhur mealladh ag an gcailín leathnocht, agus níor thug sibh faoi deara go raibh bhur bpócaí á bhfolmhú ag a páirtí “You guys were being charmed by the half-naked girl, and you didn’t notice that your pockets were being emptied by his partner”
Bhí “Scéal fá Dhá Chathair” á chumadh ag Charles Dickens “A Tale of Two Cities was being written by Charles Dickens” (or rather “authored, composed” – I used the verb cum!/cumadh)
When the action is definitely completed, we use the participle (the -the/-te/-tha/ta form).
Tá an cath briste orainn ag na trúpaí naimhdeacha “We have been defeated by the enemy troops” (in Irish we say, “the battle has been broken on us by the enemy troops”)
Bhí sibh meallta ag an gcailín leathnocht “You guys had been charmed by the half-naked girl”
Bhí “Scéal fá Dhá Chathair” cumtha ag Charles Dickens “A Tale of Two Cities had been written by Charles Dickens”
There is a definite difference between the Irish construction Bhí sé déanta aige and the English construction It was done by him. They do not mean the same. The Irish construction is basically the exact equivalent of the German “situational passive” or Zustandspassiv – es war von ihm gemacht/getan. It means that it had been done previously, but the prevailing situation is that it was not being done anymore, and only the results existed at the time referred to. Thus, tá an cath briste orainn is best translated with the English perfect: we have been defeated; and bhí an scéal cumtha is a pluperfect: the story had been composed.
Now, of course, you want to ask, how you exactly translate into Irish such a construction as A Tale of Two Cities was written by Charles Dickens. This English construction does signal that the act of writing has been completed, but it also stresses the act, not just the result (the Irish situational passive only stresses the result). My short answer is: you don’t. You say instead Charles Dickens wrote A Tale of Two Cities, or ’twas Charles Dickens who wrote A Tale of Two Cities. Thus:
Chum Charles Dickens “Scéal fá Dhá Chathair”.
Ba é Charles Dickens a chum “Scéal fá Dhá Chathair”.
Those who learnt their Irish mainly from contemporary non-native sources will ask me, what about *Cumadh “Scéal fá Dhá Chathair” ag Charles Dickens then? I have always recommended against using ag agents with the autonomous verb. Here is why.
It is to be admitted that during different periods, there have been attempts to use an agent with the autonomous verb. My illustrious fellow countryman Anders Ahlqvist once pointed out to me that Old Irish used the preposition that was the cognate of as (‘out of’). Diarmuid Ó Sé has in his article in Ériu in 2006, Agent Phrases with the Autonomous Verb in Modern Irish, surveyed this kind of constructions. It is suggested by Niall Ó Dónaill in his indispensable dictionary that le was used in older literary Irish – basically, in Early Modern Irish and later attempts to approximate classical style – for this purpose.
However, Ó Sé is of the opinion that such constructions as cailleadh X le Y “X was lost (killed) by Y” do not suggest that Y killed X, but rather, that Y was the reason of X being lost or killed – i.e., Y didn’t wield the weapon that killed X. A typical construction is cailleadh an laoch le bean álainn – i.e. a beautiful woman was the reason why the hero was killed (in the classical or postclassical examples used by Ó Sé, bean still had the dative form, so cailleadh an laoch le mnaoi álainn would be closer to the actual style, but you get the picture). The idea here is more like “he was lost/killed through a beautiful woman, a beautiful woman was his undoing”.
In fact, this usage of le is still common in such constructions as cailleadh le hocras é “he starved to death” (“he was lost through hunger”) and even the active construction fuair sé bás le hocras (“he got death through hunger”). I would also suggest that le is possible as a kind of agent preposition when the agent is not personal, but, for instance, a force of nature, so that it is difficult to say whether it is an agent or a reason: scoilteadh an spéir le tintreach (“the sky was split by a lightning”).
Another possible agent preposition is ó, which is used in Connacht Irish in such expressions as pósadh ón sagart iad “they were married by (actually from) the priest” (this example comes from Tomás de Bhaldraithe). Moreover, Ó Sé points out that Tomás de Bhaldraithe also found such gems in Connacht as this:
cén fáth nach múinfidís ó Ghaeilgeoir í? “why wouldn’t they teach her from an Irish-speaker?” (the meaning intended is “why wouldn’t they have her taught by an Irish-speaker?”)
Now, I feel very tempted to recommend these usages of ó to you, but I guess I must refrain from that. This ó usage will not be understood outside Connacht, and it is so uncommon that I have never encountered it in native literature or folklore.
Then that ag. Such constructions as goideadh an t-uisce beatha ag an druncaire “the whisky was stolen by the drunkard” are encountered in non-native literature, in bad newspaperese and in officialese, but I must say that they are really grating if you have acquired your Irish through the study of native literature and folklore. In fact, while folklore elicited from terminal speakers (i.e. speakers who aren’t regular users of the language anymore, and whose grasp of the language is loosening) does tend to exhibit unacceptable Anglicisms and solecisms, this ag usage is practically non-existent even in such material.
Diarmuid Ó Sé notes that they are sometimes found in texts written by native speakers especially of Munster background, when they try to sound refined and literary. Myself, I have found an abundance of ag agents in Dónall Mac Sithigh’s book Fan Inti, which is a Munster native speaker’s account of traditional boat-making. However, the book is in this respect very exceptional. It is my impression that these constructions are one of the most obvious differences between “good Gaeltacht Irish” (which you acquire, in Finland, by reading books written by native speakers as well as by reading folklore) and “that horrible school Irish”.
Diarmuid Ó Sé also notes that the ag is most often attached to the autonomous past tense, typically not to other tenses. He says this is a “genuine syntactic restriction” and refers to Edward Keenan’s and Matthew Dryer’s article on Passive in the world’s languages, which suggests that there is a connection between perfective verb and the need for an agent construction. Irish past tense is, according to him, basically an aorist, i.e. a verb form that refers to a completed (perfective) action. I agree on that, but I find Ó Sé’s explanation a little long-winded.
My impression is that this use of the ag agent has originated in non-native Irish and especially in direct relative clauses, which can be ambiguous: if we say an t-oifigeach a mharaigh an saighdiúir, does it mean “the officer who killed the soldier”, or “the officer whom the soldier killed”? You are tempted to use *an t-oifigeach a maraíodh ag an saighdiúir if the second interpretation is correct.
However, there is another way to avoid this ambiguity. You see the fact is that you are allowed to use indirect relative clause in such occasions. Then, write out the correct pronoun and use the indirect relative particle. Like this:
an t-oifigeach ar mharaigh sé an saighdiúir “the officer who killed the soldier”
an t-oifigeach ar mharaigh an saighdiúir é “the officer whom the soldier killed”
These usages are found in native folklore, and while they are not very common, they are found in all dialects, and sometimes even when they are not necessary. For some reasons though, they seem not to be taught at school, which may be one reason why those ag agents are so common in school Irish.