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.
Ireland’s tax agreements with multi-national companies have been ruled by the court’s to violate EU laws in the past, so Ireland I think is an outlier. 130 countries signed on, there is clearly broad international support. Ireland even agreed in principle, they just think 15% minimum is too high, but their entire pitch to multi-nationals is as a tax haven. If they join a minimum tax pact, there’s no real reason to HQ in Ireland anymore.