If the question is whether the noun era can be modified adjectivally by three words in succession, two of them adjectives (modern wireless) and the third itself a noun (networks) used as an adjective, then the answer is Yes, that is permissible.
If the question is whether the preposition "a" is the proper determiner here, then the answer is probably no; "the" would be better:
Multi-sensor fusion and consensus filtering are two very fascinating
research subjects in the modern wireless sensor networks era.
We are speaking of the contemporary era, "now", so we use "the modern wireless sensor networks era". "A" would suggest that there can be many, possibly recurrent, eras characterized by "wireless sensor networks", and even if that were true, the presence of the word "modern" means "this one we are in", so we use "the".
Some writers would revise your phrase on stylistic (not grammatical) grounds:
Multi-sensor fusion and consensus filtering are two fascinating
research subjects in the era of modern wireless sensor networks.