nua-aimseartha doesn't mean "fashionable". Not all modern things are fashionable, and not all fashionable things are modern.
seanaimseartha can be interpreted as "old-timey". English doesn't have an equivalent word "new-timey", but if it did, it would be nua-aimseartha in Irish.
No. aimseartha relates to time, and nua-aimseartha means "up-to-date", "newfangled", modern.
"fashionable" doesn't imply modernity (old things or styles can become fashionable again, for example, but they don't become modern). faiseanta would be used of clothes or style, nósúil of an idea or method.
The closest equivalent in English is the "timey" part of the somewhat informal adjective "old-timey"
The "fangled" part of "newfangled" probably isn't a helpful comparison, but "new-timey" isn't used in English.
"time" is one of the meanings of aimsir
aimseartha is necessary - nua doesn't mean "modern".