Gabriel. Speaking as someone who has already been in similar situations, I'ld like to give you a few pointers to consider and maybe act on. I can only guess on what your plan is.
1. Don't overestimate the public use of the internet as a reference base. Even though some of us on this board use it as a competitive right-arm, many don't.
2. Get in contact with a CEO or Director, and ask them how they would approach a mailshot arriving on their desk. I think if they are honest they will tell you where it goes.
3. Don't overestimate the perception that businesses are looking for good value, especially the established ones. There is a large element of traditionalism in Ireland, whereby they stay with historical suppliers for certain goods and services.
4. You may get verbal agreement from many, but work on a safe contingencey of how many will actually hand over the money.
5. Don't underestimate your competitors (if any). You may have more competitors than you actually think. If you are promoting a web based media, then your clients are going to pull back from another form of media. Hence a new competitor. They will hold out longer than your 10k€. They usually have a crisis contingencey for that situation.
6. Don't underestimate the cost of advertising in Ireland. If I pay you, I want to see how you are going to promote that portal.
8. If you are approaching businesses, have figures for them which indicate "added value". Otherwise you are the same as the last person they spoke to.
You will find that friends and family are supportive and positive, but I just feel that it helps for the negatives to be emphasised.
Regards "If I was considering it how does one go about approaching someone like Denis O'Brien?"
Hid in a bunker on an Algarve golf course and wait for his ball to land there. Have your speak then.
Best of Luck!!