There is indeed a rule which tells you when you need to insert an epenthetic vowel. We have exactly the same thing in Irish, so I'll tell you the rule for Irish. In a consonant cluster made up of L, N or R before B, BH, M, MH, F, G or CH there will be an epenthetic vowel (with the exception of NG, which is pronounced as a single sound); but if there is a long vowel immediately preceding this consonant cluster, then there's no epenthetic vowel. It should be the same in Scottish Gaelic, perhaps with some very subtle variation.

Ha! Yes, Steven, we can definitely call them helping vowels. :) This is a great list to practice! Thanks for the help!

Can't we just say "helping vowel" on a site like this! It's for general language learners. I always get called up by non-Scots for my pronunciation of "girl", which is of course " girul" The fuaimreag chòmhnaidh or "helping vowel" is rarely written, but occurs in spoken language after a short vowel between the following pairs of consonants. LB LBH LCH LDH LFH LG LGH LM LMH MCH MGH ML MN MR MS NB NBH NCH NFH NGH NM NMH NPH RB RBH RCH RFH RG RGH RM RMH SCH The list is not exhaustive. Some examples of words: Alba aimsir ainm Murchadh dearg gorm arbhar morbhair Colbhasa Uilbhinnis falbh! ealdhain anmoch ban-charaid /banacharaid ceann-bhaile / ceanna-bhaile seana-bhaile seana-mhàthair = seanmhair Sgarbaidh (Scarba) Ceann Loch Gilb (Lochgilphead) tilg marbh etc. The helping vowel exists in a weaker form in words like: Glaschu, iomradh