You don't get a security text when activating your SIM.
https://community.48.ie/t5/The-Knowledgebase/Getting-Started-How-to-Activate-your-48-SIM/ta-p/53099
I presume you mean when porting your old number to 48?
https://community.48.ie/t5/The-Knowledgebase/Getting-Started-Move-Your-Number/ta-p/53100
If you're not receiving the number porting security text then you'll have to contact 48 support.
See the link in my signature below or email them at support@48months.zendesk.com
While you're here, check out our "Featured Topics" below.
Our Troubleshooting Guide and Knowledgebase contain help and support articles and popular questions to help you as a 48 customer.