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You mean when you get it? Usually in cases of -oi- in a stressed syllable or when -aoi- is followed by a bh, dh, gh, mh (I think that's all). It's not very predicable. The worst pair is coire vs coire, one with the grinning o and one without. Goid has it, bois hasn't... it's a funny one.(although I'm still not 100% clear on where grinning O arises).
I don't blame you. The way Celtic linguistics writes these is NOT helpful. They have the tendency to take the orthographic lnr, capitalise according to strength and then add an apostrophe to indicate slenderness, so you get L L' l' l N N' n n' R r' r. Which is really little else than restating the orthography with a minor clue to strength. I know why they do it - as not all varieties of Irish and Gaelic have dealt with the 4-4-3 system the same way (Harris, for example, has L L' l' l in that notation but in hardcore IPA, that's actually l?? ? l l?) in that system but it does little to telling you what these actually are phonetically. I took me about a month to pin each down, in particular the Irish authorts.I had always thought that the slender/broad distinction held even for weak N, L, R.