diff --git a/intl/unicharutil/util/IrishCasing.cpp b/intl/unicharutil/util/IrishCasing.cpp
index 6c66ff517101..17407cc38ca8 100644
--- a/intl/unicharutil/util/IrishCasing.cpp
+++ b/intl/unicharutil/util/IrishCasing.cpp
@@ -208,33 +208,36 @@ const uint8_t IrishCasing::sUcClasses[26] = {
   kClass_letter
 };
 
+uint8_t
+IrishCasing::GetClass(uint32_t aCh)
+{
+  using mozilla::unicode::GetGenCategory;
+  if (aCh >= 'a' && aCh <= 'z') {
+    return sLcClasses[aCh - 'a'];
+  } else if (aCh >= 'A' && aCh <= 'Z') {
+    return sUcClasses[aCh - 'A'];
+  } else if (GetGenCategory(aCh) == nsIUGenCategory::kLetter) {
+    if (aCh == a_ACUTE || aCh == e_ACUTE || aCh == i_ACUTE ||
+        aCh == o_ACUTE || aCh == u_ACUTE) {
+      return kClass_vowel;
+    } else if (aCh == A_ACUTE || aCh == E_ACUTE || aCh == I_ACUTE ||
+               aCh == O_ACUTE || aCh == U_ACUTE) {
+      return kClass_Vowel;
+    } else {
+      return kClass_letter;
+    }
+  } else if (aCh == '-' || aCh == HYPHEN || aCh == NO_BREAK_HYPHEN) {
+    return kClass_hyph;
+  } else {
+    return kClass_other;
+  }
+}
+
 uint32_t
 IrishCasing::UpperCase(uint32_t aCh, State& aState,
                        bool& aMarkPos, uint8_t& aAction)
 {
-  using mozilla::unicode::GetGenCategory;
-  uint8_t cls;
-
-  if (aCh >= 'a' && aCh <= 'z') {
-    cls = sLcClasses[aCh - 'a'];
-  } else if (aCh >= 'A' && aCh <= 'Z') {
-    cls = sUcClasses[aCh - 'A'];
-  } else if (GetGenCategory(aCh) == nsIUGenCategory::kLetter) {
-    if (aCh == a_ACUTE || aCh == e_ACUTE || aCh == i_ACUTE ||
-        aCh == o_ACUTE || aCh == u_ACUTE) {
-      cls = kClass_vowel;
-    } else if (aCh == A_ACUTE || aCh == E_ACUTE || aCh == I_ACUTE ||
-               aCh == O_ACUTE || aCh == U_ACUTE) {
-      cls = kClass_Vowel;
-    } else {
-      cls = kClass_letter;
-    }
-  } else if (aCh == '-' || aCh == HYPHEN || aCh == NO_BREAK_HYPHEN) {
-    cls = kClass_hyph;
-  } else {
-    cls = kClass_other;
-  }
-
+  uint8_t cls = GetClass(aCh);
   uint8_t stateEntry = sUppercaseStateTable[cls][aState];
   aMarkPos = !!(stateEntry & kMarkPositionFlag);
   aAction = (stateEntry & kActionMask) >> kActionShift;
diff --git a/intl/unicharutil/util/IrishCasing.h b/intl/unicharutil/util/IrishCasing.h
index a4b2edc29206..9c95c4d83283 100644
--- a/intl/unicharutil/util/IrishCasing.h
+++ b/intl/unicharutil/util/IrishCasing.h
@@ -101,6 +101,14 @@ public:
 
   static uint32_t UpperCase(uint32_t aCh, State& aState,
                             bool& aMarkPos, uint8_t& aAction);
+
+  static bool IsUpperVowel(uint32_t aCh)
+  {
+    return GetClass(aCh) == kClass_Vowel;
+  }
+
+private:
+  static uint8_t GetClass(uint32_t aCh);
 };
 
 } // namespace mozilla
diff --git a/layout/generic/nsTextRunTransformations.cpp b/layout/generic/nsTextRunTransformations.cpp
index db955571a382..03ddac7bef07 100644
--- a/layout/generic/nsTextRunTransformations.cpp
+++ b/layout/generic/nsTextRunTransformations.cpp
@@ -276,6 +276,8 @@ nsCaseTransformTextRunFactory::TransformString(
 
   bool capitalizeDutchIJ = false;
   bool prevIsLetter = false;
+  bool ntPrefix = false; // true immediately after a word-initial 'n' or 't'
+                         // when doing Irish lowercasing
   uint32_t sigmaIndex = uint32_t(-1);
   nsIUGenCategory::nsUGenCategory cat;
 
@@ -331,6 +333,24 @@ nsCaseTransformTextRunFactory::TransformString(
         }
       }
 
+      cat = mozilla::unicode::GetGenCategory(ch);
+
+      if (languageSpecificCasing == eLSCB_Irish &&
+          cat == nsIUGenCategory::kLetter) {
+        // See bug 1018805 for Irish lowercasing requirements
+        if (!prevIsLetter && (ch == 'n' || ch == 't')) {
+          ntPrefix = true;
+        } else {
+          if (ntPrefix && mozilla::IrishCasing::IsUpperVowel(ch)) {
+            aConvertedString.Append('-');
+            ++extraChars;
+          }
+          ntPrefix = false;
+        }
+      } else {
+        ntPrefix = false;
+      }
+
       // Special lowercasing behavior for Greek Sigma: note that this is listed
       // as context-sensitive in Unicode's SpecialCasing.txt, but is *not* a
       // language-specific mapping; it applies regardless of the language of
@@ -349,8 +369,6 @@ nsCaseTransformTextRunFactory::TransformString(
       // position in the converted string; if we then encounter another letter,
       // that FINAL SIGMA is replaced with a standard SMALL SIGMA.
 
-      cat = mozilla::unicode::GetGenCategory(ch);
-
       // If sigmaIndex is not -1, it marks where we have provisionally mapped
       // a CAPITAL SIGMA to FINAL SIGMA; if we now find another letter, we
       // need to change it to SMALL SIGMA.