Many ISPs just resell the eircom wholesale ADSL package so will be subject to the eircom line speed limit. Some have their own infrastructure so may not be.
On the other hand...
I was on UTV 2Mbps for a couple of years. They had an offer on 3Mbps upgrade so I opted for it. My broadband stayed on 2Mbps. I queried this with them. They told me that my line was only capable of 1Mbps. I told them that I had been on 2Mbps for a couple of years no problem. The downgraded me to 2Mbps. I don't really trust those broadband line checks to be honest!