In order for anybody to prove their ownership of a property, it's pretty much essential that the necessary transfer (in this case from A to B and from B to A) would *at least* go past the revenue commissioners and be stamped at whatever the rate is; in other words, neither the registry of deeds or the Land Registry (depending on the systems the house(s) are on) will accept them.
Of course if either A or B in the future decided to sell their dwelling in the future, it'd be a lot easier if they were the registered owner.
This isn't legal advice, per se, more common sense than owt else; if there's a hit to take on tax, then however unpalatable it is, it makes more sense to take it on the chin *now* than to defer it to the future where surcharges and such like might make their presence felt. It's quite possible that a purchaser in the future would demand (rightfully) that there are no tax issues affecting the property and thus both parties would do well to be fair and square ASAP.
to the OP, and in concurrence with Mitzy, it'd seem that stamp duty will figure, unless you're moving into a terribly small house...the smart thign to do would be that your husband should get the house from his mum and pay 1/2 the rate of stamp duty on the entire value and *THEN* transfer half that interest to his spouse, upon which transaction there's NO stamp duty. It takes longer, but it's better than paying full stamp duty on half the value of the house and half rate on the other half...
all that, btw, is perfectly legal...it's not even a loophole!