"'S e do bheatha, a bhràthair."
Translation:You are welcome, brother.
No - not directly. It is to do with lenition. The singular possessives 'mo', 'do' and 'a' (my, your, his/her) trigger lenition but the plurals 'ar', 'ur', and 'am' (our, your, their) do not.
Because we are not being respectful to the brother (similar age/status etc) we use 'do' here and therefore have to lenite beatha to bheatha. But with our venerable grandfather, we are respectful and so use the 'you plural' form of the possessive (your = ur) and therefore drop the lenition.
For more information see here: http://www.akerbeltz.org/index.php?title=Possessives_and_syllabic_structure_or_Ar_n-Athair_a_tha_air_n%C3%A8amh
(Akerbeltz is a major Gaelic Guru - so keep his pages book marked).
If you remember French from school, you'll recall using 'tu' for individuals, young people and close friends and 'vous' for groups, older people and authority figures. It's the same deal in Gaelic... with the additional gotcha of singular possessives triggering lenition.