Britain’s leaders are facing increasing calls to take action to loosen abortion restrictions in Northern Ireland after the Republic of Ireland’s landmark referendum in favor of doing so, but complex political realities may make quick action difficult. Prime Minister Theresa May is being asked by some legislators and activists to take steps that might lead to liberalization in Northern Ireland now that Ireland has voted overwhelmingly to repeal its constitutional ban. Once new laws are put in place by Ireland’s parliament, Northern Ireland would be the only region in the U.K. and Ireland to ban abortions. Its strict law prohibits...