Fiesty Sheep has some good ideas.
If you like place names such as Ailsa (which is pretty) there is also Iona, Shona, Rona and Vaila, Eriska (all islands; the last two Norse in origin rather than Gaelic). There is also Lorna (said to have been invented by novelist RD Blackmoore for 'Lorna Doone', and based on the place name Lorne, in Argyll). And Morven.
Nothing to frighten the horses with these:
Ailis (sometimes also spelled Ailish/Eilish )
Mairead
Kirsty (Gaelic = Ciorstaidh)
Fiona isn't Gaelic but was very popular in the West a couple of generations ago
Fenella /Finella (version of Gaelic Fionnuala)
Flora (Gaelic Floraidh, pronouced Florreeh)
Marsaili as suggested by previous poster: MARH-sah-lih
Annag (the 'g' is more like a soft 'kh') = Annie
Davina (originally Davidina) was used in the past in Gaelic-speaking areas; similarly Andrina, Malina and several others based on male names)
Maura (English-language version of Irish Maire)
Senga (but use with care; it's sometimes said to be 'agnes' backwards, and used to describe a female 'Ned', but is more likely to come from a Gaelic word meaning slender.)
Mirren/Merren (Scots)
Elspeth (Scots)
Ishbel (Scots)
Greer (Scots surname)
Grizel (Scots - looks grim but when said nicely, it's pretty: Grih-SELLE); Annabel/Amabel was originally Scots
Marion (Scots, from French): like Fiona, was popular a couple of generations ago