Translation:Does he clean or does his mother clean?
It's not actually an /n/, however (the eclipse of /d/ is). Instead, it's the sound at the end of he English word - /ŋ/
Basically, what's happening with eclipses is that a voiced stop gets nasalized (thus /b/ to /m/, /d/ to /n/, etc.), a voiceless stop gets voiced (/p/ to /b/, /t/ to /d/, etc), and a /f/ gets voiced.
an bhfuil is only for questions that involve "are/is". Otherwise an acts as a general 'question particle' when it comes before a verb (with eclipse) that means the sentence is a question. So think of it more like "does" in other cases.
An bhfuil sé ceart go leor? -- Is he alright?
An ritheann sé -- Does he run?