I have been on both Three (Same as Virgin Media) and Meteor and found them both equally rubbish to be honest.
I mainly used in West Dublin, City Centre and Midlands. Both were fine in the city, but once outside urban areas they really struggle.
I moved to Vodafone last year and have not looked back. You do pay a premium for the improved service, to me it's worth it.
On the basis that Virgin use the Three infrastructure, I'd ultimately look to peoples experiences on Three... take a look over on the Talk To Us (Three) section of Boards to see just how unhappy many of their customers are with unreliable coverage. Personally, I let them sometime ago given how bad their network was and I would not touch any operator relying on their network with a twenty foot barge pool !
€5 is cheap, but what they will also do is refresh your current virgin media contract to 12 months, so you lose the chance to switch providers for a better price.
Virgin mobile can't receive any automated messages, eg automated texts from banking apps when adding payees, messages from couriers letting you know when your delivery is scheduled for etc. Big thread on boards.ie about, can't post the link but google "No automated SMS messages inbound" and you'll find it. I'd stay away from virgin mobile.