The only Kirstys I've ever know were only children - how annoying!
I don't think anything long, fussy or overly girly would go.
Kirsty, and:
Amber, Amy, Ariel, Annie, Ayla, Bethan, Bonnie, Cerys, Ciara, Connie, Cassie, Callie, Darcey, Ellen, Eilidh, Elodie, Erin, Eden, Ebony, Flora, Farrah, Faith, Gemma, Hannah, Heidi, Halle, Isla, Iona, Iris, Jasmine, Jessie, Kaitlyn, Lara, Lexie, Lucy, Martha, Maisie, Macie, Morgan, Megan, Nicole, Orla, Paige, Robyn, Rosie, Seren, Shona, Sarah, Sophie, Sadie, Tegan, Tara, Tessa, Zoe, Zara.