Where are you going and for how long as both have a bearing on your questions?
You won't lose your number unless you don't buy membership for several months - I think it's about 9 or more but check the Terms and Conditions if you need to know:
Terms & Conditions
You may be able to receive calls without any membership but to make any calls you need membership.
However while abroad membership allowances don't cover any usage.
When abroad you need cash credit to cover the cost of outgoing and incoming calls and outgoing texts.
The call costs are detailed here:
http://www.48months.ie/membership-plans/going-abroad
If you are in the EU then EU roaming calls and texts add-ons may be more cost effective - especially because the calls add-ons makes it free to receive calls.
http://www.48months.ie/membership-plans/going-abroad
EU roaming add-ons are no use outside of the EU.
Data will not work while abroad so you are restricted to wifi where available.
If you have an iPhone then be aware of this before you travel:
http://community.48months.ie/t5/Need-Help/Iphone-6-no-network-when-overseas/m-p/32727#M11081
Depending on where you're going and for how long it may be more cost effective to use something like Skype/WhatsApp over wifi or even get a cheap SIM locally but then you won't be on your "normal" number.
Hope this helps.
