This a purple-leaved form of False Shamrock (Oxalis triangularis 'Cupido'). The species is native to Brazil, Peru, Bolivia, Paraguay, and northern Argentina. There are numerous forms with varying amounts of green and purple on the leaves. The cultivar name, 'Cupido' seems to be applied to plants that have both a lighter and darker shade of purple streaked on each leaflet. The purple-leaved forms of Oxalis triangularis have also been incorrectly named Oxalis triangularis ssp. papilionacea, but they are not truly a distinct subspecies.
The word Shamrock derives from an Irish phrase, seamair óg which translates as 'clover, young', so presumably, the original 'true' shamrock was a species of Clover (Trifolium sp.). Some have gone so far as to identify Red Clover (T. pratense) as the 'true' shamrock.
Nowadays, in horticulture, it is much more commonly used as a name for various ornamental Oxalis species.