The 23-year-old defender, now capped three times by the Republic of Ireland, was a part of England's set up at junior level, appearing for four different age groups before being persuaded to switch allegiance.
"Ciaran has a massive future," said Lambert, "and the more caps he can accumulate, then great.
"It's hard to say if England have missed a trick, or not. It wasn't until recently that I found out Gabby Agbonlahor qualifies for Scotland.
"England do have good centre halves. To have Ciaran is definitely Ireland's gain - there are not many left-footed centre-halves about, so that gives him a great advantage.
"I saw him play against Greece and I thought he did really well. It's there for him to go and do it and have a big career now for Ireland."