Can I keep an unused Foyle Metro mLink ticket on my phone until I need it?

Yes. Translink states that purchased Foyle Metro mLink tickets remain in the mobile wallet until activated.

Once activated, a day ticket is valid for the remainder of its applicable travel day on scheduled Foyle Metro services. Activation is the key difference from mobile products that start immediately upon purchase.

Practical advice: Do not activate the ticket long before your first journey unless you intend to start its validity then.