I've never heard of an employer taking legal action against a departing employee, but that's not the only issue. Ireland is a small world, and you really don't want to leave your current employer on bad terms. It would be very unusual for a new employer not to accept a one-month notice period. If the new employer loses you as a recruit, how long will it take them to find someone else - and won't that person most likely be on 1 months notice as well?
Do you have any outstanding holidays that you could take as part of your notice period, so you could start with the new employer in week 3 or 4 while you are technically on holidays from your old employer?
I'd set the expectation with the new employer that it's definitely going to be 1 month and haggle with the old employer to see if you can get out quicker than that.