C: Cén canúint a labhraíonn siad ar an nGealach?
F: Gaeilge na Mumhan! An oldie but a goodie!

Note: If you do not understand this joke, highlight this: (Gealach is the Irish for Moon and 'Mumhan' is pronounced similarly to 'moon')

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Char thuig mé 'n jóc ar dtús, cha bhfuaimneamsa "moon" agus "Mumhan" ar an dóigh chéarna

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Watch out, Lughaidh, you're so sharp you might just cut yourself.

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Char úrt mé ach an fhírinne!
fuaimneam [ˈmũːənˠ]

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

The point you seem to be missing (apart from the fact that it is impolite to spoil someone else's joke) is that [ˈmũːənˠ] is within the spectrum of possible English pronunciations for "moon". So the joke is on you.

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Visit Dublin some time and ask a few people what is in the sky at night. Irish isn't the only language where we can pronounce things differently. Anyway, I am greatly looking forward to your offering on the joke front and I promise I'll be kind.......

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Níl's agam tuighe a bhfreagrann sibh mar sin... char úrt mise féin ach nár thuig mé 'n jóc ar dtús siocair nach bhfuaimneamsa an dá fhocal ar an dóigh chéarna, tá 's agam go ndéantar, i gcanúintí áiríd. Sin an méid. Ní gá fearg a bheith oraibh. Tá sibh a' freagairt fá mar a mhaslóchainn sibh! Tógaigí go bog é!

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

We are all friends here and I did promise to be kind......
(Anyway, I have altered the explanation to include the word 'similar' to allow for slight variations of pronunciation.)
And, most importantly of all, I have mastered how not to leave big spaces between quotes!

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WARNING: Intermediate speaker - await further opinions, corrections and adjustments before acting on my advice.
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), agus Gaeilge an Chaighdeáin Oifigiúil (CO).