This paper explores the characteristics associated with marriages between Roman Catholics and members of other religious denominations in Ireland before the Great War. Using the entire digitized returns of the 1911 population census, we find that such marriages were relatively rare, occurring in less than one percent of total marriages. Some of this infrequency can be attributed to ethno-religious hostility-especially in the north of the country. However, we also show that the rarity of intermarriage reflects local marriage markets, as non-Roman Catholics living in communities with fewer coreligionists were more likely to intermarry. We examine the individual characteristics of partners in these marriages, looking at the religious denomination of their children, their decision to marry out, and their fertility behavior. Our findings illustrate how the frequency of intermarriage reflects historical levels of intolerance, but only after local marriage market conditions have been accounted for.