Quickdraw once the bug gets you it is hard to stop, you may know everything below but I cant tell so I am not patronising, just throwing out possible leads.
From the few Megaws on familysearch.com I think the interest might focus on one of the following, my guess being the killinchy line.So again I would focus on chutch records for killinchy etc
HUGH MEGAW Birth: 20 MAR 1869 , Antrim, Ireland
Parents:
Father: JOHN MEGAW Family
Mother: MARIA MC SHANE
GEORGE MEGAW Christening: 22 JUN 1824 Presbyterian, Killinchy, , Down,
Parents:
Mother: AGNES MEGAW Family
GEORGE MEGAW Christening: 28 OCT 1822 Knockbreda, Down, Ireland
Parents:
Father: THOMAS MEGAW Family
Mother: NANCY PATTERSON
I looked at emerald ancestors and found a spelling McCaw even McWha
On a fee paying website
www.ancestryireland.com Marriage Record MEGAW HUGH 1856 Banbridge
Civil Marriages MEGAW HUGH 1856 Down
Marriage Record MEGAW HUGH 1857 Banbridge
Civil Marriages MEGAW HUGH 1857 Down
Marriage Record MEGAW HUGH 1892 Banbridge
Civil Marriages MEGAW HUGH 1892 Down
Sometimes Down means the civil parish of Down ie an area around Downpatrick.
From an irish gravestone inscription service, a fee paying sister site of UHF is
www.historyfromheadstones.com/Newmills Pres Tullyhish Ballyngarrick Upper MEGAW
Returning to PRONI
Cowan and Megaw papers
D/1413
There appears to be quite a few Megaws who were botantists/archaelogists etc see Google
Try
www.lennonwylie.co.uk and ask its search engine to do Megaw
See
www.genfair.com/shop/pages/uhf/page07.html and search for Megaw{family history}
As always I read the question too fast from Ros davies again
Hill Head, freepages.genealogy.rootsweb.com/~rosdavies/ Comber Map Reference 21: 43/66 Farm;4km SW Of Comber, The 6inch OS map of townlands for the 1830s and 1860s are in existence.
I really must stop. All the best, Jim