There are a number of sources that say flatly, female leprechauns do not exist. Lady Anna, who decended from the Irish at Tripod.com states flat out "female leprechauns do not exist." Wisegeek notes "there are no female leprechauns noted in folklore, begging an obvious question. Then again, faeries don't follow the same rules as humankind. Chiff.com wryly states, "There is no mention to be found of female leprechauns in traditional Irish legend, so as to how they came to be .. your guess is as good as mine." I could go on, but you get the drift.
On the other hand, there are also sites that do describe female leprechauns. Google claims that radiohead.com has a message stating, "Female leprechauns live in caves - ask a leprechaun 29th Mar 2008." However, when I clicked on the link, it didn't go anywhere useful. Answerbag jokes that female leprechauns are Leprechicks. Interestingly, the avatar of the person with this answer, NoWayJose, looks a lot like one of our compatriots avatars:
Kris Dueck, some blogger from somewhere, went on holiday to Ireland, with the express purpose of "fall[ing] in love with a female leprechaun." He later claims, "As my tour group stared at the magnificent Polna Brone Dolmen burial tomb, my eyes scanned the shadows in search of magical eyes. At first I wasn't sure it was a lady, but her red beard was silky soft, and freshly manicured. A word about female leprechauns; unattractive. I stuck my hand down the hole, and felt her long soft facial hair. Then she bit me. "
The St. Patrick's Day of Central Texas
Parade & Festival rhetorically asks, "Did you know that all "real" female leprechauns are beautiful?" Then they claim: "It's a fact!"
So, clearly, the sources are all over the place on this one. That's the trouble with Google, as we all know.
OK, so I'm a geek. I work at a University. I used JSTOR to find a scholarly article. Blech! In Fairies and the Folklore of Disability: Changelings, Hybrids and the Solitary Fairy, by Susan Schoon Eberly, published in Folklore, Vol. 99, No. 1 (1988), pp. 58-77, she seems to be saying that a possible origin for all these little critters (brownies, sprites, pixies, leprechauns, etc) is children with congenital diseases: cleft lips, downs syndrome, and many other things too awful to mention. There are two pages of pictures of children with deformed heads. Maybe these children were tossed out, and left to fend for themselves, and that's why people would put out milk and crusts at night, to assuage their guilt. In different areas, people might make up different names, or maybe each different shape was given a different name.
OK, so that's probably more than you wanted to know. Suddenly, this isn't fun any more. It was so sad, in the past, that deformed kids were tossed out, and seen as evil or worthless.
My opinion? I think we can make it up however we want to. These are magical, nonexistent creatures anyway. You want a female leprechaun? I give you one. You think it's more intriguing that there are none? I'll give you that. It's your call.