Looking at distributions of Cotters from historical surveys such as Griffiths & the 1901 census, there are four clusters of Cotters in Ireland. DNA results have so far grouped these into two unrelated groups. Noone descended from the Clare cluster has tested yet.
Those of east county Cork and probably Kerry may have entered the Isles with the Anglo-Saxons, from Frisia (Netherlands). They may have held the surname since family names were established around 1200 AD. The forename Garrett, of Germanic origin, is indicative of this group.
The west Cork group have likely been in Ireland for around 4000 years & are genetically close to Mahony and O'Leary who have strong Cork origins. Indicative forenames of this group reach back to Gaelic times: Jeremiah (Diamaid), Timothy (Teigue), Cornelius (Conchobar).