Rob
I've looked at that death cert for Susan in Jan 1909. She shows as born in 1908 (4 months old at her death). Parents confirmed as William Christie and Margaret Gibson, so they had arrived in Linlithgow by late 1908/early 1909 - they show at Bridgend
Susan's birth is not showing on Scotlands People (SP), so likely the family came over after her birth between Oct 1908 to Jan 1909.so you will have to order one of the later births via SP or the GROS. There is the on line cut off for births on SP of 1907, anything after this date has to be ordered.
Now in respect of finding out where in Ireland they hailed from, it is incredibly hard to normally find this info out, however a couple of routes open to you:
1. Scottish birth certs from mid 1860s include the date and place of parents' marriage
So, any of their children's birth certs from Scotland will include this information. You know that Susan doesn't look to have been born in Scotland. There are only 8 Christie births showing in Linlithgow on SP between 1909 - 1919 (avoiding anything from 1920 when you know Henry married which would confuse matters). I would probably look to order either of the Mary births (corresponding with the deaths that Sicknote found). Linlithgow births for a Mary Christie show on SP in 1912 and 1915.
2. The next census due is the release of the 1911 census, rumoured for early release at some point next year. A lot of people of Irish origin simply stated Ireland for a birth place, however, you may be lucky and find that William and Margaret show an actual Irish County for their birth place.
Best route at this stage would be to order the birth of one of the girls, Mary, that died young.
Monica