London (CNN) Same-sex marriage is now legal in Northern Ireland, bringing the province in line with the rest of the United Kingdom, where it has been legal since 2014.

Following the Westminster vote, Stormont held five votes before a slim majority returned a vote in favor of legalization in 2015.

However, this was immediately contested by the Democratic Unionist Party, which tabled a "petition of concern" -- a mechanism that can be used by lawmakers in Stormont to flag up motions before the assembly that could fall across sectarian lines, and so must have both cross-party and cross-community support.

Because of Northern Ireland's unique status and history, Stormont operates on a power-sharing agreement, requiring parties representing both republicans and unionists communities to govern together.