Xyrillians had an unusual physiology. They did not have a metabolism based on water or breathe an oxygen-based atmosphere. In fact, their high-pressure atmosphere caused a hallucinogenic effect to most species, and as a result, most other humanoids needed to be weaned onto and off their atmosphere to avoid undesired effects. When traveling, Xyrillians grew edible vegetation on the bulkheads of their ships, and had eel-like creatures swimming in tanks. Xyrillians could pass electricity via tactile contact, though this was painless and could even be pleasant.
Xyrillians also had dermal plating, which they found necessary for detecting each other's moods. Apart from this plating and the fact they were slightly shorter than the typical Human, there weren't many obvious physical characteristics that distinguished Xyrillians from Humans.
Xyrillian males usually carried infants to term in a pouch on the upper chest. They served only as hosts, and the offspring only contained genetic material from the mother. Impregnation could occur with only minor physical contact and could be unintentional on the mother's part. Humans and Xyrillians were genetically compatible, as Commander Tucker discovered when he was impregnated by a Xyrillian female. He also developed a nipple on his wrist, but it is unknown if this occurred with Xyrillian males as well or if it was a non-standard reaction of inter-species reproduction with Humans. (ENT: "Unexpected")
These aliens were referred to in a one-paragraph story outline for "Unexpected" that was written in a memo regarding story ideas, sent from the ENT writing staff to Brannon Braga (on 4 May 2001, when the story had the working title "Conduct Unbecoming"). The aliens were not yet named and were entirely female. They were also described as "weirdly sensual creatures."
In the script for "Unexpected", the Xyrillians were described as "exotic humanoids, glistening with moist, scaly skin."
According to Star Trek: Star Charts ("United Federation of Planets III") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), there was a star system named Xyrillia, which might be the home system of this species, in the Alpha Quadrant. It was a binary system with a A-class star and a F-class star. In the late 24th century, it was an independent system.
StarTrek.com described the Xyrillians as an "Alpha Quadrant" species.