If I were you I'd ask eir to convert your account to PAYG and keep it alive for the first month, just to be sure there aren't any issues with Lyca (data speed, signal, banking verification codes etc). The only drawback is having to make calls/texts with another number for that month.
Once you put the Lyca SIM in the phone, it'll have that SIMs phone number (089...). This won't have any impact on the eir phone number until you port it to Lyca using the Lyca SIM in conjunction with this page...
Note the times for porting...
Business time for weekdays between 08:00 to 17:55
Business time for weekends between 10:00 to 15:55
Don't forget that you don't have to buy a bundle, topping up by 15euro gives 15GB and all calls (but no text) and keep the 15 top up.