Still haven't found this place, but made a little progress, on several McDonnell records from the church in Collinstown the McDonnell's address is Strillstown, Collinstown seems to be their local church as the girls are getting married there.
So with this spelling (L x 2) I found a report of drainage works from 1848 at Strillstown Mill, the heading above it is Lough na Saggart District - Counties Meath and Westmeath.
At least one of the McDonnell brothers was a Farmer and Miller in 1901 but it was Michael not Patrick.
Had a look at the old maps and lots of mills but none seems to be named.
I have her parents names now, which was my original reason for looking for Strillstown but I'd still like to find it as it's where they were from and just because it annoying not to solve it.
Maybe some day someone will see this and have the answer, clearly the name has changed, Streamstown sounds good but it's a bit far away from Collinstown.
Cheers.