On every country having the same tax as Ireland, that is kind of what happened with the pact. Except that some countries won´t accept it, thus leaving room open.
First, there was no pact, Ireland didn't agree to it, so the rest can get that 15% and stick it where it belongs.
Second it not about every country having the same tax as Ireland but more on Ireland raising its tax to other countries levels.
I think the fact that one of the few countries not adhering to the global lower limit is Ireland, is pretty much significative for the direction of the trade after being signed: vertical archiving in the dustbin