There are many ways of studying the effects of protection on the industrial structure and trade flows of the protected economy. In this paper, we do so by comparing certain features of the economies of Northern Ireland and the Republic of Ireland during the mid-'sixties. At that time, the Republic was a highly protected economy and consequently would be expected to be much less specialised than Northern Ireland, both in the sense of having a less diversified range of industries and, at an intra-industry level, of having higher shares of output exported and higher import penetration of domestic markets. The results prove to be remarkably consistent with these expectations. They suggest very strongly that, even by 1971, the Republic had a long way to go before achieving the levels of specialisation reached in Northern Ireland.