Hi Emilie!
I'm sure he'll love the surprise! What an amazing gift!
This 10 days itinerary seems great, with longer stops in Kerry and Connemara to rest, so you won't feel like you are spending too much time packing or driving.
Since you got 3 nights in Killarney (2 full days to explore the Ring of Kerry), you can maybe spare time in
West Cork (let's say start in Kinsale, then Mizen Head and perhaps Bantry Bay on the way to Killarney). It might seem a bit much though so pick a place or two along the road, I always recommend spending more time in a few places than trying to rush to see it all.
For the day spent driving between the Cliffs of Moher and Clifden, I'd suggest stopping by
the Burren, Galway (or Salthill nearby for lunch) and then follow the coastal stretch of the Wild Atlantic Way passing by Roundstone, Dog's Bay and the Skyroad until you reach Clifden. Alternatively, you can drive inlands through the boglands (Maam Cross, Derryclare Lough) and see the coastline the next day (maybe make your plans according to the weather).
Are there any activities (hikes, built heritage or culture) that you and your partner are interested in? Maybe you can spare half a day trip for a boat trip (the
Aran Islands or the Blasket Islands for instance) if that's something you are interested in. Has your car rental agency a different drop-off option? (don't forget your credit card)
A bientôt,
Antoine