"Come in, Fraser."
Translation:Thig a-steach, a Fhrìseil.
Because the vocative particle 'a' only goes in when the following name/noun in the vocative does not start with a vowel sound (because two vowels together is often bad, why we use 'an' before vowels in English). Fionnlagh is f+vowel at the start which when lenited to Fhi the fh bit goes close to silent leaving the start of the name with a vowel sound, so the 'a' isn't put in front. But for the other name it's not F+vowel but F+consonant, Fr so when lenited Fhr doesn't start with a vowel sound, so the vocative article 'a' stays in. Does that help?