Seabrook is an English surname. From the Old English river name which meant 'slow-moving', + Old English broc 'stream'.
Buckinghamshire
It's not of Gaelic origin.
Its an English form of the Gaelic name Mac Leoid.
Gaelic
If a surname of English origin it would remain the same. If a surname of Irish origin it would be Mac Siurtáin (son of Siurtán, Jordan), a Gaelic name assumed by the d'Exeter family.
Gaelic (Scottish or Irish).
The surname Barry is Irish and English of Welsh origin. In Ireland it is the anglicized form of Gaelic Ó Beargha 'descendant of Beargh', a byname meaning 'plunderer'. In England, of Welsh origin: it was a patronymic from Harry, the medieval English form of Henry.
Although the name is found in Ireland most are apparently of English or Scottish origin.
McMillan is a Scottish name, an English form of Gaelic Mac Maoláin. In Scotland the usual spelling is Macmillan.
It is Goisdealbh which is the origin of the surname Costello(e).
This name is of Scottish Gaelic origin. It comes from a Scottish surname and a place name which probably meant "garden of hollies".
The surname Roach is not named after a bug. In English it is a topographic name for someone who lived by a rocky crag, from Old French roche later replaced in England by rock.
It is a Scottish surname meaning 'son of Coinneach (Kenneth)' and is spelled MacCoinnich in Scottish Gaelic.