It is very likely this street was called after Hugh Percy, the third Duke of Northumberland, who was Lord Lieutenant in Ireland 1829-1830. The committee had approved of Messrs. John Alexander & Co. Naming a new street leading from Shankhill Road to the Falls Road paralleled to Dover Street and Northumberland Street, " Percy Street," also two cross streets running from Morpeth Street into said street, "Kendal Street," and "Tyne Street" respectively.Ulster Examiner and Northern Star - Thursday 02 April 1868. Percy Street which was named by Belfast Corporation in 1868, Northumberland Street is close by. The Belfast And Province Of Ulster Directory 1858-1859 lists several addresses for Northumberland Street with some vacant ground.
The Irish language form only applies to part of the street.