When you hover over "boy," one of the translations is "mbuachaill."
Is there a reason for this in a different grammatical context, or is it just a spelling difference?
Yes, the reason is introduced a little later. In Irish words beginning with some letters get an "uru" in certain circumstances, such as when preceeded by the definite article "an".
They'll teach you about it shortly.
What's up with the capital N?
Which capital N do you mean?
I wish I could speak fluent Irish.