The stress is on the first syllable which is pronounced like "ale." The next syllable is pronounced just like the English "is." The last syllable is short, sounding like "sed". ALE-is-sed.
I was provided this information by a native Manx expert in Manx Gaelic.
I can hear the American public school teacher saying, "Ok, ee-la-sed, your turn to read." Sorry. Fact is, if you want people in the US to call your child Ailasajh, you'll just have to spell it like that (or something similar).
I was provided this information by a native Manx expert in Manx Gaelic.