just try to visualise the ramayan kind of scenes where the demons say "My Earth" [mi-rth] and laugh in a similar fashion.
mirth sounds close to myth..which are usually unbelievable and hilarious and makes us laugh
the best MIRTH I've had was when my wife gave BIRTH to my baby
mithr.. friend .. merriment and laughter
MIRTH=MARY+BIRTH. For Christians, the BIRTH of Christ to MARY is cause for MIRTH.
mirth = Mother + birth; Mother has given the birth of a child so she is happy.