Often we are asked, "What makes a piece of lace Irish Crochet?"
Venetian Gros Point Lace
Irish Crochet is a form of lace created in the 19th century to mimic opulent European needle laces, such as 17th century Venetian Gros Point.  In Ireland in the mid-1800's, the Potato Famine caused widespread poverty and hunger.  Hundreds of Irish peasants learned to make crocheted lace, and the new lace trade saved countless families.  People saved up their lace money and sent family members to America to escape the famine, or stayed and survived in Ireland on that money.  Thread and a hook were all they needed to provide relief to their loved ones.
Traditional Irish Crochet is made up of cotton thread, but sometimes silk or linen thread was used.  Thread sizes vary and many of the antique pieces are made with thread as fine as our modern sewing 
Irish Crochet
thread.  The finer the thread, the more delicate the resulting lace.  
To create the lace, individual motifs are made, usually with a much thicker packing or padding cord to make a more three dimensional shape. Common motifs are flowers and leaves, including shamrocks.  The maker crochets along the thicker cord, pulling and smoothing the stitches as she/he goes. The motifs are then arranged on a paper or fabric pattern (for a collar, cuffs, or other item), and joined together with crochet stitches in a somewhat freeform style. The maker goes around each shape and creates a mesh background to contrast the more solid motifs. The mesh is made with chain stitches and can be embellished with picots or Clones Knots. Finer mesh or thicker bars can be used to create lighter or more dense areas in the lace.  Often, certain families became known for their specialty motif, and that was all they created.  Motifs were usually brought to a central location where they were combined with different motifs, and then crocheted together by others.
Left:  motifs laid out on a pattern.  Right:  motifs joined with size 80 thread in Clones Knots.
(worked by Kim at Maire Treanor's workshop at Lacis, in 2011)
Irish Crochet lace trims or inserts are made of square trellis motifs, usually with a shamrock or rose in the center. These are then joined end to end to create long pieces of trim. This type of Irish Crochet lace is generally dated later, usually from the 1900’s.
Left:  Shamrock Trellis Motif, by Kim.  Right, antique trim with added bottom edge embellishment, courtesy of adtjmd