A former colleague of mine wrote a History of Art thesis in Trinity in 1999 or so, either M.Litt or M.Phil, on Ashlin. I'm sure it would have some info you require. I'll send you her email by private message.
Also, the office of Ashlin and Coleman still exists, though without family connections to Ashlin or Coleman, I believe. They might be able to help re old drawings, company archives etc. A quick google gives two addresses: 36 Pembroke Road, D.4, or 1 Grant's Row, off Lwr Mount St, D.2, and an email (possibly out of date) of firstname.lastname@example.org