Leine's are specific to the Irish. Unfortunately, there is little definitive knowledge of Irish dress prior to the 1590s - mostly we just have English records bad-mouthing them :(
A correctly made leine would be considered authentic for any late-period Irish persona.
What is this? This is The Garbindex - a guide to making garb for recreationists (ie how to sew clothes that will make you look genuinely medieval, when 'playing' with such groups as the SCA, doing LARP - or even making something really genuine for drama.)
Use these resources to find resources that will help you create clothing that looks great - and real!