Allen has two quite distinct origins, one Scots-Gaelic, the other French. Ailin, meaning "little rock", is the root of the Scottish name, originally MacAllan. The first recorded arrivals bearing the Scottish name came in the fifteenth century, as hired soldiers ("gallowglasses") imported to Donegal by the O'Donnells, and the migrations of the following two centuries brought many more. In other cases, the surname derives from the old Breton personal name Alan, which in turn came from the Germanic tribal name Alemannus, meaning "all men". The same root provided the modern French name for Germany, Allemagne. Followers of the invading Normans were the first to carry the Breton version of the name to Ireland. Irish families bearing the name may be of either origin, though the fact that two-thirds of the Allens are to be found in Ulster - they are especially numerous in counties Antrim and Armagh - suggests that the majority are of Scottish extraction.

Allen households by county in the Primary Valuation property survey of 1848-64.
Click on a county name for a parish breakdown(paying).
Antrim 151
Armagh 114
Belfast city 59
Carlow 5
Cavan 11
Clare 10
Cork 104
Cork city 27
Derry 68
Donegal 35
Down 132
Dublin 29
Dublin city 43
Fermanagh 35
Galway 33
Kerry 8
Kildare 17
Kilkenny 10
Laois 14
Leitrim 3
Limerick 39
Limerick city 6
Longford 35
Louth 8
Mayo 6
Meath 53
Monaghan 17
Offaly 15
Roscommon 10
Sligo 38
Tipperary 50
Tyrone 103
Waterford 36
Westmeath 65
Wexford 30
Wicklow 25