As I understand it, ea is an old neuter pronoun (it). We see it in * Is ea / 'S ea and ní hea Roughly: "That's it" and "That's not it". It is used with copular constructions as a fronting tool, to add stress. So Is fear maith é is the baseline, but to stress the subject you could have Fear maith is ea é* (He is a GOOD MAN).
Compare Is duine deas é He is a nice PERSON
Is deas an duine é He is a NICE person
Duine deas is ea é He is a NICE PERSON (It's a NICE PERSON that he is)
Is maith an duine é It's a GOOD man (that) he is
If I've got this wrong, I'm sure someone will help us all by jumping on it.
Use of the copula can be complex; this page provides thorough explanations. The structure of the sentence above puts the emphasis on fear maith, in the same way that the tone of voice in “He is a good man.” would in English — that is, he is a good man, rather than whatever other description of him is being considered.
I can't follow the link in the app, but if you respond to this, I'll get an email, which will link me to the webpage for this discussion, and from that I'll be able to follow the link.
So, apologies for the convoluted process, but I hope you'll respond because right now this exercise makes no sense to me.
Its literal translation is "Man good is it he". If you want to get your mind around this literal translation you will have to get your head around the Coupla. The coupla lets you shift words around for emphasis. Your example is wrong because with "ea" dropped you need to remove emphasis and right shift "Is fear maith é", as you left shift and move the "is" over you must add the ea in, Fear maith is é, is just wrong. In one of the dialects (Munster) people often default to this way of saying it, and its so common you could argue that the emphasis might be lost, however for the other two dialects this movement is purely for emphasis "A Good man, he is" vs "He is a good man", to practice just try both forms, when you learn a new word "Doctor", do "Is dochtúir é", "Dochtúir is ea é", but never ever (dochtúir is é), and just reinforce it over and over with new words and new examples till its second nature.
According to wikipedia:
The vocative case is a grammatical case which is used for a noun that identifies a person (animal, object, etc.) being addressed,
in English, the "man" in "My good man" is in the vocative, but the "man" in "Good man!" isn't, because "Good man!" isn't a form of address.
Fear is in the vocative when it is used as a form of address, like "Sir" or "Mister" might be in English ("Hey Mister, is this yours?" - A fhear, an leatsa é seo?).