Opting into TENNER just gives a free "bonus" texts to other Vodafone users when topping up 10 EUR on more. You could top up 15 EUR without texting TENNER and have 15 EUR credit to purchase the data bolt on without the free texts. It encourages people to top up with a certain minimum amount and offers different rates when opted in to top up "bonus".
https://www.vodafone.ie/pay-as-you-go-plans/charges/

I usually wait to activate a new local sim until I arrive in destination country. Some sims can be activated outside of the home network while others can't. Those which can activated while roaming, beware of the charges of incoming calls from recycled number or accidentally leaving the data on. I believe Irish sim cards activate fairly quickly, so doing it during arrival to gate shouldn't be difficult.

If you activate it outside Ireland, I'd suggest topping up when you arrive or just before departing for Ireland to preserve the credit balance.

After you power up the phone in Ireland, also check the APN settings is set correctly to live.vodafone.com.
http://prepaidwithdata.wikia.com/wiki/Ireland

If you want to keep any remaining credit for future travel, be sure to opt out of any recurring charge plans prior to leaving Ireland. I don't believe Vodafone.ie data bolt on is a recurring charge but I've learned via experience to check that detail if I want to preserve any remaining credit for a local sim. Recurring PAYG charges will go through even if a sim isn't active on the local network.