I've always found music, both concerts and ceilidhe or sessions, in Skye and in Stornaway, Lewis. (Elsewhere in the Western Isles it isn't as easy to find the music as there isn't the pub culture.) Check the Stornaway Gazette and the West Highland Free Press (and "Lochaber News" etc, the appropriate local papers according to which area you are in) for announcements and adverts, also local radio stations. There's often music around the Gaelic college, Sabhal Mór Ostaig in the Sleat/Teangue area of Skye (near the ferry at Ardvasar)