It looks like you are a fairly low-end user. I was similar with an iPhone 4 from O2 a couple of years ago, and also had no interest in upgrading the phone. (A decent phone to me is an indispensable commodity, but not something to ooh and aah about, and waste money on inessential features or expensive upgrades). I briefly used O2 prepay, but their data policy is a joke and it was a waste of time having a smartphone when your data's switched off. I moved to prepay with 48months.ie and haven't looked back. €10/month for 300 mobile any-network minutes, free texts, 60 land line minutes, and 1 GB of data. I've rarely used more than a fraction of any of the allowances in a month, and I prepay a year at a time so I never have to even think about phone charges except for the odd bit of roaming or calling non-standard numbers.
If you're a heavier user there's a heavier package for €20/month with unlimited mobile minutes and texts, and 5GB of data. You can buy extra add-ons if you run out of any component during a month. The only thing is you might have to be "generous" about your age when signing up, however 48 themselves encourage this so it's no big deal. As far as I know they use O2's network. There's been one or two niggly annoying things about them, but nothing worth complaining about at a tenner a month! (Disclaimer: I have no connection with them other than as a customer).