I have always used Morr-i-gan - sometimes when talking to Irish scholars and have not been corrected. Just because Ree-on is the correct usage for Rioghan in Nua-Gaeilge or Modern Irish doesn't mean it is the correct usage in Sean-Gaelige(Old Irish). AFAIK the "gh" would have been sounded in Old Irish. Pronunciation shifts in diphthongs and letters are quite common in languages as they evolve - Nua-Gaeilge has evolved from Sean-Gaeilge, through Mean-Gaeilge(Middle Irish) to the Irish we know today. It does not actually make sense to prn
Morrigan as Mor-ee-an in Modern Irish.
I do agree it's silly to wilfully pick and use a wrong pronunciation if you know better, just because it's pretty. I would be hypocritical if I said this didn't drive me crazy sometimes, actually. However as Indialove pointed out even in
Ireland the same name can be pronounced several different ways.
However it also makes me feel uncomfortable, to say the least, when people take as Btn's pronunciation of a name as blind fact. Yes, Btn's database is a wonderful resource and is usually right but(and I am only going by Irish names) is not always right. The entry for
Liadan, for example makes me frustrated. So just because it says Mor-ee-an in the database doesn't make it right.
So in other words I prefer Morr-i-gan and use it in preference and have always assumed it to the right prn. It's the pronunciation I have heard in
Ireland without exception.