Woody
The fact that you're calling an Irish mobile means that any method is going to be more expensive than we'd like. I'm pretty sure VOIP will not be an option (it would require a high-speed, 3G, internet connection at both ends and, for obvious reasons, mobile phone companies do not like VOIP on their phones).
If you used an Irish SIM (or your father used a UK SIM) one or other of you will be paying considerable connection charges (although the EU is forcing these costs down).
Calling an Irish mobile from Vodafone seems to cost me 12p/min plus VAT (on a business tariff ... personal is probably different). Bizarrely (and I defy anyone to explain this), this is cheaper than calling a UK mobile out-of-bundle (i.e. once all inclusive minutes are used).
Vodafone seem to offer an International package (http://www.vodafone.co.uk/personal/price-plans/managing-my-costs/calling-abroad/index.htm) for £5.00 per month which will bring rates in line with the business tariffs.
There are plenty of companies offering cheap international calls (search 'cheap calls ireland' or similar). You dial an 0870 number (between 8p & 15p per minute) followed by the international number. Call quality is not always perfect as many are using the cheapest connections available.
