The "c" of "caife" is formed at the back of the throat (that "k" sound is a "glottal" or "gutteral" stop or "plosive"); "gcaife" is simply the voiced version of "caife:" both the "g" sound and "k" ("c") sound are "glottal" (or maybe it's "gutteral") stops (or "plosives"). Following the same pattern, "bportán" is the voiced version of "portán," but both the "b" sound and "p" sound are bilabial stops (or "plosives"). "Mbean" is a nasalised version of "bean," but both the "m" and "b" are voiced bilabials. The "v" sound of the "bh" in "bhfear" is a voiced labiodental fricative; the "f" sound of "fear" is an unvoiced labiodental fricative. In each case, the "mouth parts" remain in basically the same position to eclipse the consonant; if "caife" became "bcaife" (rather than "gcaife"), the eclisped version would form at the other end of the mouth, viz., between the lips rather than at the back of the throat.
b is only used to eclipse words that start with p. g is used to eclipse words that start with "c".
Irish, and other languages such as Russian and Korean, doesn't have a verb for "have". Irish uses the construction Tá X ag Y to convey the same meaning as "Y has X", but that doesn't make Y the subject of the Irish sentence, it just means that Irish and English don't use the same grammar to express that concept of possession, and X is the subject in Irish (subject of the verb tá) whereas Y is the subject of "have" in English.
In this exercise, Tá isn't being used to express some other concept, it just means "is" and there is agreement between Irish and English about the subject and object or predicate of the sentence.
All right, thanks. Still not clear to me (though of course logically there should be agreement!), but probably if I'll read up some more on that, it will eventually start making sense. So far I'm mostly doing these exercises mechanically, just constructing parts in the order I'm shown, but they don't make sense, and so I'm prone to such mistakes where I can't guess where the subject and the object go in the sentence.
"Logically" it would be impossible to translate "have" at all if there is no Irish verb that means "have".
There is no "logical" requirement for such agreement, any more than there is any "logical" requirement that "I own the cat" and "the cat is mine" should both have the same subject.