Welcome and I am so sorry you lost Charlie.
If he is registered with the IKC you may be able to just get this information from the IKC. Or from the breeder? (but please do not mention breeder names here at all, as I don't want the subject of individual breeders discussed).
The problem is literally thousands of cavaliers are born every year in Ireland -- it is Europe's puppy farm capital and there is a massive business going on, much of it very unpleaant
. Going by the names of the sire and dam, I do not think Charlie probably came from a show breeder? -- and the people who breed for the pet market often do it on a very large scale and keep hardly any records (typically when I was doing rescue, breeders kept no records even of who they homed dogs to so if we traced a chip to a breeder they did not want the dog and had no idea whose dog it might be :x ). Because of this situation unless the breeder can tell you, or the IKC will release the registration details for the litter (which still will not tell you who has the other siblings) I think you probably will find it impossible to find another person with a sibling. Most likely there were only a couple of other pups in that litter and often pups are exported to the UK.
If you are trying to trace this out of interest in finding a new cavalier for your family -- PM me privately.
There is currently only one breeder in Ireland I would recommend as she is the only person I know that is currently properly healthtesting all her breeding dogs (eg doing the critical MRI scans for syringomyelia and cardiologist (not vet) heart tests for the other major illness in the breed, mitral valve disease (heart disease). I would be happy to give you contact details.
There are online cavalier pedigree databases but these are dependent on someone having entered the dog's names and the pedigree info. Generally this is only done for show lines and they would have a kennel affix before the dog's name. The breeder would be the owner of the dam but I cannot find any breeder info for Odbughaill. Not much help I know.