Calls to Ireland using 1899 cost 1p/minute - less than 1/7 of the daytime cost of 0870. However, you need to be registered with 1899 to use their service.
From
Money Saving Expert international call checker, the cheapest way to call Ireland is currently via one of the following access numbers:
08442007575
08442004545
They cost just
0.5p/min to use (that's half a penny!) and you
don't need to sign up, register or pay any subscription charge.
To call Micro$oft, you just need to dial the following:
0844 200 7575 00353 14502113 #
The final hash is usually needed as is tells them you've finished dialling. It means they don't need a clever computer that knows the length of phone numbers in different countries.