There are a host of festivals on from March onwards (as well as Celtic Connections on now) and loads of folk events during the Edinburgh Festival fringe in August-early Sept). The TMSA (Traditional Music and Song Association of Scotland) calendar includes a lot of these dates, but is not quite yet available - check back soon to www,tmsa.org.uk
Those I would recommend include Northern Steams (Edinburgh - st weekend in April), Girvan (last weekend of April/beg of May) - same weekend as Shetland if you want to go up there, Moniaive (next weekend after Girvan) Orkney (last weekend in May) Keith (mid-June), Stonehaven (2nd weekend in July but often same weekend as Heb-Celt in Stornoway), Innerleithen (2nd or 3rd weekend in August), Auchtermuchty (around same time), Kirriemuir (1st weekend in September), Linlithgow (2nd w/e in September), Douglasdale (3rd w.e. in September).
They all have websites so Google them to find out more