Well the 0800 number you quote is answered by an Irish lady. The alternative for this is a number in Ireland which is given on the contact page which is 00353 66 97 92 719 which will not come out of your inclusive minutes.

The best way to call freephone numbers from mobiles where it's gonna cost more than an 01/02 call is to use the dial-through number 020 0222 0900. This is a normal London number which you can then go on to ring freephone numbers for free.

A search of 192.com for WESTERN UNION reveals several numbers. I think 020 8741 3639 is most promising. It's address is:

WESTERN UNION MONEY TRANSFER
UNIT 3 KING STREET CLOISTERS
CLIFTON WALK
LONDON
W6 0GY

Another number for Western Union is 020 8535 7000 mentioned here. Not sure whether it covers what you're after, but give it a try.