Verbs that are listed as ending in -igh often tend to have ú in the verbal noun. Is this some sort of umlaut process? Muster would have 'boilscig', for example, (an unlenited slender g), so maybe it's not the case, but I thought I'd ask

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__̴ı̴̴̡̡̡ ̡͌l̡̡̡ ̡͌l̡*̡̡ ̴̡ı̴̴̡ ̡̡͡|̲̲̲͡͡͡ ̲▫̲͡ ̲̲̲͡͡π̲̲͡͡ ̲̲͡▫̲̲͡͡ ̲|̡̡̡ ̡ ̴̡ı̴̡̡ ̡͌l̡̡̡̡.___

The /u:/ is a result of the typical lengthening of a historically short unstressed broad vowel before a lost fricative.
The final '-gh' of 2nd conj. verbs is made broad, then the common vb. n. suffix '-adh' is added:
boillscigh > boillsciugh+adh = boillsciughadh
athruigh > athrugh+adh = athrughadh

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(SL)(GM)

Makes sense. Thanks

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__̴ı̴̴̡̡̡ ̡͌l̡̡̡ ̡͌l̡*̡̡ ̴̡ı̴̴̡ ̡̡͡|̲̲̲͡͡͡ ̲▫̲͡ ̲̲̲͡͡π̲̲͡͡ ̲̲͡▫̲̲͡͡ ̲|̡̡̡ ̡ ̴̡ı̴̡̡ ̡͌l̡̡̡̡.___