Orlaith - not keen. Slightly prefer the Orla spelling but still don't really like the sound.
Saoirse - don't like this
Alva/Elva - I love Elva. For some reason I'm less keen on Alva (I realise they're the same name!) Maybe because Alva looks like Alvin to me
Sylvie - quite like it. Though I'd probably use Sylvia and shorten it. I don't think it's too old lady
Juno - it's ok. Don't think it would fit with a sibling with an Irish name
Margot - don't like this. Not especially old lady, just not a pleasant sound to me.
Erin - really like this
From your list I'd definitely go with Elva followed by Erin
My all time favourite Irish girl name is Sorcha. I also like Fionnhuala, Niamh and Caoimhe
Non Irish (well some might be, I'm not sure!) suggestions:
Ariel, Aria, Ayla, Astrid, Aphra, Bronte, Brodie, Cerys, Cleo, Dulcie, Effie, Etta, Enya, Fallon, Faith, Giselle, Hazel, Heidi, Harmony, Honor, Ida, Iona, Juliet, Katrina, Kayla, Lyra, Lyla, Lara, Lana, Lila, Livia, Leona, Larissa, Lorelai, Mila, Mala, Mirren, Mallory, Miriam, Naomi, Nia, Nyah, Polly, Rosalie, Rowan, Rowena, Ramona, Romy, Romilly, Robyn, Sadie, Sorrel, Saffron, Saskia, Tegan, Taryn, Verity, Vanessa, Xanthe, Zia