An iyalorisha is a woman initiated into the priesthood of a particular orisha and empowered to initiate others into it. Everything structural said of the babalorisha applies: she is initiated to one orisha rather than to Ifa, the house she heads is a lineage, those she initiates are her children in the religion, and seniority within the house is reckoned by initiation rather than by age.
The office is not a female version of a male one. It is the same office held by a woman, and in the tradition's own terms the pair are counterparts. That distinguishes orisha priesthood from the Ifa priesthood, where the standing of women initiates, the iyanifa, is genuinely argued over between houses. No comparable argument attaches to the iyalorisha: women have held orisha priesthoods for as long as there is a record of them, and in several cults they hold them predominantly.
Certain cults are women's cults outright. The Osun priesthood at Osogbo is led by women, and the Iyalode and Yeye Osun titles that appear in the festival there are women's offices in a women's hierarchy. So is the Gelede complex, which is performed by men but is held to honour the spiritual power of women and of the ancestral mothers.