"A kitten! Thank you, father."
Translation:Piseag! Tapadh leibh athair.
Because you use the plural / formal form to address your father (as a show of respect - he is older, and also your father). You can use it with friend, too (I did, and probably would, since it reads to me more as a polite way to address a stranger than something I'd say to an actual friend).
No, grammatical number, not gender.
Google "T-V distinction" for the full treatment. Leat (le+thu) is singular, and used informally, so people who are either your social equals, intimate, or social inferiors. Leibh (le+sibh) is plural, and used formally, so any kind of formal address, and when speaking to social superiors (including family members more senior than you, and strangers, especially if they are older than you).