I am creating an INLA mesh using the coastline as my boundry. I am getting some extra spaces or triangles on the right side of my mesh. I want to setup the coastline as my boundry of the mesh, so I don't have any triangles on the land. In the second picture you can see the coastline I want to use as boundary for my mesh. 
Below is the code I used to create my mesh
#Building the mesh
mesh.loc <- SpatialPoints(as.matrix(coo)) # point coordinate
mesh <- inla.mesh.2d(loc=mesh.loc, max.edge=7e3, cutoff = 0.1, crs=4326) # max.edge= 7e3 to specify the length of my triangle
plot(mesh, main="") # plot the INLA mesh
points(coo, pch=20,col="Dark Green") # green dot locations of the data frame points on the mesh
create a polygon with coastline to constraint the INLA mesh
lon = c(-4, 2.5, 2.5,-4) #lon for the FOF
lat = c(55, 55, 58, 58) #lat for the FOF
Poly2_Coord_df = data.frame(lon, lat)
poly_2 <- Poly2_Coord_df %>%
st_as_sf(coords = c("lon", "lat"),
crs = 4326) %>%
st_bbox() %>%
st_as_sfc()
FOF_bound_2<- st_difference(poly_2,coast) # crop the FOF polygone with the UK coastline
FOF_bound_pol<-st_as_sf(FOF_bound_2,crs=4326) # setting my coastline boundaries as the shapefile
