Complex sentence: In 1890, a birth census of English families in Ireland was taken, finding that that there were over 269 people bearing the surname Alcorn throughout Ireland, all living in the province of Ulster, in Donegal and Londonderry.
Split into simpler sentences: In 1890, a birth census of English families in Ireland was taken. From that census, it was estimated that there were over 269 people bearing the surname Alcorn throughout Ireland, all of whom were living in the province of Ulster where the families were found in Donegal and Londonderry.