Probably Shifra. Being Irish myself I have always thought of Siofra as a stange name for a person, as it means "changeling". Changelings are usually bad things in stories, aren't they?
Yeah, that was my thought... I've heard, though, that the name is being used in Ireland now and has been for several years. I take it you've never met a Siofra?
I've never met a Siofra myself, but I have heard of people with the name. It mustn't be that common, and I guess it's only been used in modern-ish times, like Aisling.