Alaikumassalam wa rahmatullah,
Irsa is an Arabic and Persian name that's taken from the Greek word Iris. The most common meaning of Irsa in Arabic is the flower known as as-Sawsan al-Asmanjooni, which is taken from the Persian word Sosane Asmangooni, which in English means "rainbow lily", a type of lily whose petals have rainbow-like color gradients.
The Persian dictionaries say that the original meaning of Irsa is "rainbow", and that it is taken from Greek. Thus the modern meaning is "rainbow lily", while the original meaning is "rainbow". This name is not an Islamic name, but Muslims can use it since it has a good meaning. Irsa is pronounced [(EA)st + p(R)ay] + [(SA)nd] with emphasis on the first syllable (though you can pronounce it differently if you like). It is written as ايرسا in Persian. In English it can also be written as Ersa, Eersa, Eirsa, Eyrsa, Eirsaa, Ersaa, Irsaa.