It's called 'slenderisation' and happens in the vocative case (when talking to someone) as caran-neonach has specified. It does come up later in I think some words in the genetive and dative cases, but that's a fair way down the road. Basically, male name, that doesn't already have an i as the last vowel gets slenderised. If it doesn't begin with a vowel, l, n or r it gets lenited and if the final result doesn't have a vowel sound to start with there's an 'a' preceding the name. Female names the same but don't get slenderised at all.
I assume it was one of the tiles that you didn't use? IRN-BRU is a soft drink popular in Scotland and very popular in the Duolingo sentences. However, for reasons that are discussed here, it is easy to misread it as *IRN BRU. This means that IRN and BRU are, separately, in the set of words that exists in Duolingo, that it chooses random tiles from.