Even though the dh is generally silent, about 100 years ago at Univ, I learned, loo'-ne-sagh' (soft gutteral, as in Gaughan.)

A quick whip around (always fun) the web gives loo'-ne-sah' & LOO-nah-soo & Loo-naz & LOO-nus-uh & even loo'-ne-sar'.

One very authoritative-seeming feller says, Lughnasadh - The "g" in "gh" is largely silent, and the "dh" is an aspirated "w" type sound with something of an aspirated "h" sound.

I hope this helps.

Modern Wiccans, etc. were nor around when it was celebrated "in the field" in Ireland and I gather there is no particular continuity of oral pronounciation (as there is, eg, in ecclesiastical Latin or Hebrew) and it's bound to differ in Scotland and different parts of Ireland and may well have shiftted over time, anyway. There's general agreement on the first syllable and keep in mind that non-gutteral-language-users haven't a clue what to do with one.

Besides, how often would you use it in conversation, anyway?