On postpaid, you can get the UK add-on which would get you unlimited calls for $20 or the $3 add-on which would make phone calls $0.05/minute.
As you can see, lots of options :)
@OP: you say you call Ireland once per month...would you be calling cellphones or landlines? How long do you expect these calls to be?
If you plan on calling landlines in Ireland and don't expect the calls to go on for several hours, your cost-effective option is probably Fongo or another VOIP provider. Fongo is easy. Install the app, get a phone number, add airtime, call Ireland.
If you're calling cellphones in Europe, that can be expensive with VOIP providers (higher inter-connect fees). Koodo does not distinguish cellphones or landlines when calling Ireland, so if your usage is low (shorter calls, not hour-long calls), then it might be simpler to just pay per minute with Koodo.
If you plan on relatively high usage (on the phone for hours calling Ireland, landline or mobile), then add the UK add-on with Koodo at $20 for 1,000 minutes (I know it's for UK, but there's a trick). Then go to a free UK numbers website such as freeuknumbers4u.com. These numbers usually have high inter-connect fees, but Koodo does not distinguish these rates and treats 'em all as UK calls. Once you have the UK number, point it to your contact in Ireland as a call forward. You call the UK number, then it directs the call to Ireland. This will let you skip the inter-connect charges to Ireland, and work out of your UK 1,000 minutes with Koodo.
There are other providers such as Vonage and PhonePower with similar price-points with bigger buckets of minutes (2k minutes and 5k minutes), but instead of calling directly through your Koodo phone, you'd do it through a house phone connected to their device or using their app on your smartphone. This is only worth the cost and resorting to another provider if the 1,000 minutes with Koodo is not enough.