Hi Viv,
Although I have researched the Skillicorns of Lonan parish, it was a while ago, and I need your help to get up to speed.
I don't have, in my notes, any record of the marriage of Stephen Kewley and Catherine Skillicorn. The IGI has several entries for this marriage, but they are all patron-submitted. While patron submissions could well be based on family records such as Bible records, or some such, in most cases they are mere guesses, based on finding a person of the right name and age, in the place where you are looking. While this might work in cases where the name(s) are rare or unusual, this will not work at all in a place like the Isle of Man, where there are so many individuals of the same name in any given parish. It is sometimes possible to contact the people who made the submission, to see what they are basing their entry on. If you can't do that, I very strongly advise against using that entry as anything more than a clue.
At the time I did my research, I was only looking in Lonan parish, so the marriage could well have taken place in another parish ie: Onchan? Not sure from the information you gave why you think there is an Agneash connection.
I looked at your transcriptions (in Brian Lawson's index) of Catherine Kewley's (1793) and Stephen Kewley's (1784) wills/decrees, which, sadly, do not give Catherine's maiden name. I agree with you that these two wills are probably those of a married couple -- the list of children agrees.
I know there are IGI entries for the baptism (not birth) of John Kewley on 11 Nov 1753 at Lonan. Also of Philip Kewley on 30 May 1756 at Lonan. In both cases there is only one entry for each baptism which is from a parish record extraction, and those two entries only give the father's name. The entries which give the mother's name (Catherine Skillicorn) are from patron-submitted records.
IGI entries (parish record extractions) for the possible children of Stephen and Catherine Kewley follow:
1. John son of Stephen, Lonan 11 November 1753.
2. Philip, son of Stephen, Lonan 30 May 1756.
3. Jane, daughter of Stephen, Lonan 26 November 1758. According to her mother's estate settlement, she married Robert Kewin.
4. Edward, son of Stephen, Onchan 12 July 1761. Off the Island in 1784 and 1793.
5. Catherine, daughter of Stephen, Onchan, 28 July 1765.
6. William , son of Stephen, Onchan 20 May 1770. Off the Island in 1793.
The possibility of someone being able to help you will be greatly enhanced if you give as much information as you can about the family you are seeking, including, especially, sources of your information. (Don't worry about your query being too long.) For example, why do you think Catherine's maiden name was Skillicorn? Why do you think there is an Agneash connection?
Good luck with your research.
Lois