Saying the 3-4 "is such a better defense" is completely wrong. Both the 3-4 and the 4-3 have different advantages and disadvantages, neither of which makes either one inherently "better" than the other.
The main problem of the 3-4 is it is much harder on the players and much more personel dependent. You have to have a beast of a nose tackle who is responsible for both "a" gaps and will get double teamed every single play (pretty much). You need pretty big strong DT's who each cover the "b and c" gaps and need to be very strong against the run. You need DE/OLB who can rush teh passer and drop into coverage, something not everyone can do. You need ILB's who can face offensive lineman and shed blocks by guards and tackles, yet also have the athleticism to be mobile in coverage.
Do the irish have these sorts of people? Laws could be a good NT, but who will be the dt's? In a 3-4 we need two of them, instead of needed just one more to complement Laws in a 4-3. We have a few guys who can be our OLB/DE's that's not a major worry, but ILB? I love Mo Crum, but he's not a great ILB, he's an OLB in a 4-3 that we moved to the middle of the field. In a 3-4, both ILB's will be facing offensive linemen much more often than in a 4-3, and we all saw Crum struggle when facing the giants coming off the line. I don't know if we have 2 stud Interior LB's who could fit the bill.
That being said, there are many strengths of a 3-4, mainly due to the fact that teh defense doesn't know which OLB/DE is coming at them, which means it's confusing for them to call their blocks and read the coverages. The array of blitz packages and coverage schemes that can be called is more difficult to deal with than a 4-3's, but it's also more difficult to execute.
I have strong reservations about us suddenly switching to a 3-4 on a full time basis. But I guess what I'm really hoping for is for a defense that has confidence in itself and in attacking the offense, and I'll just have to trust the coaches to do their best to install a system and an attitude that makes this happen.