Greetings, and welcome to the SCA!
Harley 6149, f. 30r, Mermaid with Mirror
Because Heraldry and Names go together like Milk and Cookies.
The best resource for names that I have found is the Academy of St. Gabriel. You can submit a question to them, but they have been on vacation for some time. However, they have several articles as well as their past letters that you can consult.
They have a link to a primer on Scottish Names, [Scottish Names 101], as well as several other articles about both Gaelic (Highland) and lowland names [Link].
As for Moira MacDonald, here is what the Academy has spread across several of their archived letters:
<Moira> is an English phonetic spelling of <Ma/ire>, the Irish form of <Mary>. (The slash represents an accent over the previous letter). We haven’t found evidence that <Moira> was used during the Society’s period, so we recommend that you avoid it.
-1050, Academy of St. Gabriel [Link]
<Moira> is an Anglicization of <Ma/ire>, the Irish form of <Mary>. Unfortunately, this Anglicization appears to have been invented after the SCA period. We've found no evidence that it was used in period, and <Ma/ire> itself was extremely rare as a personal name in Ireland until the 17th century: there are a few examples from the
15th and 16th centuries, and the earliest known instance is an isolated 14th century example. The Irish considered the name too sacred for ordinary use; instead they used <Ma/el Muire> 'devotee of Mary'.  (The slash stands for an acute accent over the preceding vowel.) This was pronounced roughly MA VOOR-(y)eh, where the <y> in parentheses stands for a very, very lightly pronounced y as in <yes>. -1440, Academy of St. Gabriel [Link]
<Muirenn> is a fine Gaelic feminine name for your period . It is pronounced somewhere between MOOR-en and MOOR-yen, where the OO is pronounced as in <moo>. This name is not related to <Moira>, a modern
English phonetic spelling of <Ma/ire>, which is the Gaelic form of <Mary> . The slash in that name indicates an accent on the preceding letter.
-1709, Academy of St. Gabriel [Link]
For MacDonald, in period this simply meant “son of Donald,” not “part of the MacDonald clan” [Link – St. Gabriel]
The SCA Heraldry website also has an article on feminine Scottish names [Link].
I’d also encourage you to consult the herald’s table at an event, or get in touch with your local/regional/kingdom herald for help. There are “book heralds” and “court heralds” – you want one of the former for help with research. I’m not a herald – I’m just a librarian. 😀
You could also post to the SCA Heraldry Chat Facebook Group to get some guidance as well.
All of that being said, don’t feel too pressured to have a surname nailed down before you go to your first event. A first name is enough – I still haven’t submitted my paperwork for my “more period than locative” surname. People will generally call you by whatever you introduce yourself as, and first names are always easier to remember (at least in my experience).
Good luck to you, and welcome!