I'm surprised that they need this and can't just port your number without more info...
If they insist on getting the ICCID then the last 13 digits of this are printed on your 48 SIM.
They should be able to figure out the remaining leading 6 digits.
But if not then contact 48 support and ask them for the full 19 digit ICCID.
Hope this helps.
Edit: see also here: http://community.48months.ie/t5/Other-Help/SIM-card-19-digits/td-p/42777