There is an ATM on Inishmore now. When I was there first I lost a bet because there wasn't. Back then the bank opened twice a week. Anyway, the ATM in the Spar Supermarket which is open from 9am to 8pm Monday to Saturday and on Sunday from 10am to 5pm.
I really enjoyed this part of the fact sheet I recently got from here.
The Bank of Ireland provides the following service to the islands, weather permitting:
Inis Mor: Every Wednesday during the year
Wednesday & Thursday – June/July/August
Inis Meain: 2nd Tuesday of every month
Inis Oirr: 4th Tuesday of every month
It's good to see that life still has its own pace on the Aran Islands :-)