Judy,
This Scottish site provides advice on genealogical queries in Scotland:
http://talkingscot.com
This site below shows you where the surname Gillan/Gillen was found in Ireland in the mid 1800s. It speculates that the name originates in Tyrone or Connaught. It is found along the north Irish coast quite a bit which could also point to Scottish connections. Many people in those coastal areas did go back and forth over the years. There were few good roads in Ireland so overland journeys were slow and difficult whereas water was the equivalent of a freeway, and much trade and business used the seas. (The modern ferry from Ballycastle in Co. Antrim to Campbeltown in Kintyre takes 40 minutes. Ballycastle to Port Ellen on the Scottish island of Islay takes 1 hour. Gives you an idea of how close the places are. Even in the days of oars and sail they weren’t big journeys).
thttps://www.johngrenham.com/findasurname.php?surname=Gillan
If you visit a graveyard in Islay you will find many of the same surnames as in graveyards in Ballycastle, Cushendall and places like that. It doesn’t take a great deal of research to realise that these folk went back and forth all the time. They also spoke the same language (the Gaelic in the Glens of Antrim is almost identical to that spoken in Islay or Argyll, but different to the dialects in other parts of Ireland). Robert MacAdam wrote in 1873:
Even yet the Glensmen of Antrim go regularly to Highland fairs, and communicate without the slightest difficulty with the Highlanders. Having myself conversed with both Glensmen and Arranmen, I can testify to the absolute identity of their speech (Ó Baoill 2000: 122).
Richardson noted that many Highlanders had settled in deserted lands in Inishowen and Antrim after the Williamite conquest, and as they spoke no English, were supplied with Gaelic-speaking ministers from Scotland:
In the Northern Parts of the County of Antrim, which being also deserted by the Irish, upon the landing of the English army near Carrickfergus in 1689, many families from the Western Isles of Scotland, who understood no other language but Irish, settled there. At their first going over, they went to church; but not understanding the divine service celebrated there, they soon went over to the communion of the Church of Rome, only for the benefit of such exhortations, as the Popish priests usually give their congregations in Irish. And when they were asked the reason, why they did so? They said, ‘It was better to be of their religion, than none at all’ (Richardson 1711: 28-9).
Ownership of the island of Rathlin (just off the coast from Ballycastle) was reportedly disputed between the Scots and the Irish many hundreds of years ago. The matter was resolved by investigating whether snakes would live there. (There are no snakes in Ireland but Scotland has plenty). Reportedly they took some snakes to Rathlin but they died. So clearly the island was Irish.