Not quite, but it's close!

William the Conqueror lived from 1028 to 1087. The oldest extant title in the Peerage of England is the Earl of Arundel, which was created in 1138 and survives as a subsidiary title for the Duke of Norfolk. The line seems to have been passed uninterrupted to the descendants of the original Earl, although several times the line jumps to a nephew or an illegitimate son, and one Earl was attainted (but his descendants had their title restored).

That's just England, of course. In Wales, most of the native nobility from this time is no longer extant. There are certainly those who pursue romantic fantasies regarding long-lost descendants of the last Welsh-born Prince of Wales, but those are mostly just that -- fantasies. There are plenty of peers today with Welsh titles, but they were created long after Norman times.

In Scotland, we're in luck! The title Earl of Mar is attested as far back as 1014, which would put it before William the Conqueror was even born! However, there are some caveats attached to this claim. First, Scotland has a system of nobility that is similar but not entirely identical to the English system. Historically, there were often cases when the two kingdoms would treat matters of inheritance slightly differently, especially after the personal union and later the merging into a United Kingdom -- see the case of William Alexander, Lord Stirling, who was screwed over by one of those issues and then became a great hero in the Revolutionary War (on the American side). Second, the original tile for the Earls of Mar was "Mormaer," not Earl, because Scotland didn't call people "Earl" until a bit later. And third, there was a pretty big dispute over whether or not the heirs to the Earl of Mar were correctly recognized, and the title was technically dormant for a period of several hundred years before the rightful heirs resumed the title. So the descendants of the original Earl (Mormaer), assuming that the genealogy as accepted now is finally correct, have not continuously held the lands or title accorded them since the time of William the Conqueror. Not to mention that the "official" date of the title's creation is 1404, according to an Act of Parliament.

Now, talking about Ireland might be stretching it, because remember that William the Conqueror was only ever King of England, even though he did sorta put a toehold in Wales and Scotland and Ireland and also those countries have been or currently are united with England under the same monarch. Ireland will put us into some trouble because the original pre-Norman titles are super muddled. There were nobles who were created during the Norman period as Anglo-Normans making their way in Ireland, and then there were nobles created into the English peerage after England finalized its conquest of Ireland. You could argue that the traditional leaders of the Irish clans, those that survive, qualify for this question (and their descent, if correctly reckoned, is positively ancient) -- but that gets into political matters, especially since the titles are no longer officially recognized by the Republic of Ireland. (Also, Scotland has those, too.)